My 6 year old daughter is a nightmare on playdates

No more playdates.

Meet up at the park or soft play etc. instead.

My son did this, or went completely the opposite way and got every toy out the cupboard and it was complete carnage.

I stopped doing play dates at home and stuck to meeting at the park.

(Age 20 he was diagnosed ASD, but that's by the by).

Make the play date shorter? 3hrs is ongoing when you are young.
Does she behave the same on neutral territory, like the park?

Yes mine did this a bit, he was very intense and hyped everything up in his mind and then reality hit. Can’t remember how we got through it really, lots of reinforcement of behaviour expectations, we had a code word I would use which meant he was spiralling and needed a reset. He’s 9 now and doesn’t have a huge circle of friends but gets on very well with those he does have. I was similar actually and hit my friend stride about year 10! And now have many many many friends.

I don't think she will learn any skills if you just stop the playdates- could you do some intensive mediation at these moments and maybe start a reward chart to help incentivise her to master her emotions and give a prosocial response? You could validate whatever emotion she is feeling but also tell her firmly that the thoughts she's having which are causing the emotion are not based in fact- help her mentalise the other child and their intentions

Christ on a bike she’s 6 - it’s a phase for whatever reason.

OP just give the play dates a rest. They obviously aren’t working right now.

My son can be like this and it is so difficult. He’s 8 and aware that his friends may choose not spend time with him if it continues. It happens any time we meet up with a friend anywhere. He gets so lonely but just can’t seem to manage his unrealistic and unreasonable expectations of other people.

It’s not great and she won’t retain friends, unfortunately. Maybe do 2 hours and not after school. Take them out for a couple of hours. I had one incident like this with DD1 and it was with the child of a friend we hadn’t seen for ages. There seemed to be competition for supremacy. In your case I’d back off or do a very short activity at the weekend.

When you play with her, do you tend to let her win/follow her lead? Maybe you could hold off the playdates for a while and spend some time encouraging turn-taking and sharing when you play with her? Don't let her hog the best role or take toys that you're holding because she fancies them - she'll need to learn give and take.

I wouldn't do play dates. Arrange to meet up at the park or similar instead. Less intense.
My dd is six and she has a friend who behaves like your daughter. I didn't realise she was having a horrible time going round to play until she had been a few times and finally told me. It's not enjoyable for either child.

That must be really hard and people will likely want to stop playing with her.

I would stop the play dates, or could you invite a friend over and pre-warn your Daughter that they will be sent home if she acts up. Then actually do send the child home until she stops the behaviour? Could pre-empt it with the kids Mum so they know they may expect a call?