To ask why people do this?

I thought that was last time the same friend did the same thing? I didn't think she'd said anything this time?

If you want to keep her as a friend I think you have to have an awkward discussion at some point.
i see that you have 3 options -

1. Message friend ( with one of the previously suggested options) and drop out of this 3 way meet up and risk falling out with friend.
2. Go along to 3 way meet up but make it clear to friend that she can’t do this in the future ( again you could use some of the suggested words).less chance of falling out with friend.
3. Don’t address the issue and have a meltdown the next time the same thing happens again and you send friend an exasperated WTF message, guaranteeing a falling out with friend.

🤣🤣🤣 Yes OP, turn up with at least six extra people but keep a completely straight face if friend queries it!

Also good to live in a world where people are comfortable advocating for themselves and their own needs.

Just send her a message saying
"Looking forward to it! BTW I'm bringing DH/my mum/random other friend - he said he wanted to come when we first arranged it but I said no because I thought you wanted to catch up just the two of us, but since Joanne is coming i obviously got the wrong end of the stick so more the merrier!"

It’s rude as hell!

My hen night was ruined because of similar.

The original plan was a cocktail tasting night out in a big city with about 8 of my good friends. It was extra special because I never get to go out socially and had never been to this city. One woman who is a regular social drinker and often went to that city, was invited.

As the date approached she would come and say, “Oh by the way, I’ve invited my sister! You’ll get on great. She’s such a laugh!”

I cringed but what could I say?

Then the following week she announced that she had also invited “the girls from work” as they’d been meaning to go and it would kill two birds with one stone! But it was okay because apparently, they too were a laugh and I would get on great with them.
More complete strangers on my own Hen night….

In the end she had invited over half a dozen strangers to my event and being socially awkward (hence the lack of going out drinking before then) I just didn’t want to go and feel uncomfortable while her and her friends partied.

i gave a lame excuse of my sister and mum not being able to make the cocktail night now and I’d need to alter plans. I went for a quiet but nice meal with the rest of the Hens and cheeky friend went to the big city as she planned.

This, exactly this. As others have said if you don't say anything how is she to know your feelings about her doing this?

Make a stand, if nothing else it will make you feel empowered rather than taken for granted.

She's said shes sent a blunt message before saying she will leave them to it.

Nest time when you are arranging it, say 'can we meet just the two of us this time as I wanted to get your advice on some personal things'

She already knows you don't like when she does this so phone her and tell her you have decided to not come as you were making time for her not her and Joanne and that you don't appreciate the change in dynamic so please don't invite me out again if you are going to add extra people to the meet up.

You need to be very blunt so there's absolutely no confusion about how much you dislike this.

Of Course you say something! She changes the goalposts and was sly about doing it.

’Yes, I remember Joanne, she’s nice but I was only going to meet up with just you for a catch up. I’ll take a rain check this time and you two have a lovely time.’

I wouldn't go. I don't get why it's so hard to cancel. Surely she'll get the message not to keep doing this if you just cancel. Or maybe she wants you to cancel.

I think you should just say ‘oh yes, Joanne is really nice :) but honestly I don’t have masses amount of time and energy right now and not really up for meeting in a 3. Let me know when you’re free to meet up for a catch up you and me’ and end with a smiley emoji
something like that. Honestly it’s fine to say that. We all leave limited time and energy

Where has OP responded to her?! I've re-read all her responses twice and can't see that she's sent anything.

I had a friend who did this. What’s more the friend she brought was quite nasty to me. I dropped her. It felt as though she was saying I wasn’t interesting enough.

I think the blunt way you've now responded means you risk Joanne and your friend meeting up without you permanently and phasing you out.

I would have been gentler and said "cool 😀 will be good to see her. Can we also arrange another meet just us as I have a few bits going on that I wanted to chat about - Joanne seems lovely bur I don't want to air my dirty laundry further than I have to 🤣" and if you really had to, cancel on the day with a headache.

Then why not just do the same this time and bow out.

There’s no point fuming about this if you aren’t prepared to speak up. Saying something like ‘in that case I’ll leave you to it’ only tells your friend you weren’t bothered about coming in the first place. Saying something like ‘Could you arrange to meet Joanne another day instead as I was looking forward to the one-to-one catch up that we planned’ makes it clear how you feel.

I also think the activity planned is relevant to this. If you’d arranged a meal/drinks to catch up it’s a bit off to add someone into the event. If you’re going to see a band for example, the more the merrier.

Its friendly to Joanne! And sometimes the more the merrier... I just would prefer to live in a world where people are welcome I guess.

I was casually chatting with a friend and told her of my family holiday plans. Days later, she called to let me know they've booked to go on the same holiday down to booking the same flights!
To top it off, she invited her sister along!
She's a nice enough person, but I honestly don't want to share my family holiday with another family. It just changes the dynamics.

Lesson learnt though. I'm now very vague with my plans

What her friend has done (on purpose) isn't very friendly either

OP, you already committed to meeting friend but only when it was the 2 of you. There are really good suggestions here on how you could word it if you really don't want to go (abd I wouldn't blame you at all)

Thats absolutely fair enough too, up to you, bit then do tell your friend sounds she thinks twice about doing it again 😊

"I was looking forward to spending time just the 2 of us so I'll catch you next time. Hope you and Joanne enjoy your evening."

Poor Joanne, maybe she needs a night out or a bit of company. I'd hate to think someone didn't want to spend time with me so much they'd rather cancel on their friend.

Of course its up to you and I do think you should cancel if you really dont fancy it... but its not very friendly is it?