Depends what you had.
But if it was me, I'd send them a message saying please can you send me x amount.
I didn't realise at the time that when we paid in three parts, it wasn't taken into account that I was not paying as a couple, and therefore should have been paying 1/5 of the bill. Instead of me paying 2/6 (so effectively paying double what every other person did).
Although ideally you'd have said something at the time. You can't go back in time now.
Also if the amount you're asking for back is quite small then I'd let it slide and learn the lesson today to speak up for yourself.
You have a right to say no to things. Or just say I'd rather just pay for what I had.
If the bill was £200 and you paid £66,
Ideally you would have paid £40. So getting £26 is quite a chonk.
But if the bill was like £40 total, so you were paying £13 instead of £8, it's a bit harder to ask for £5 back. Especially when it's £2.5 per couple. And they're your friends.
Maybe also everyone didn't eat the same price of food and drinks. Maybe you had three courses and a bottle of wine and the couples just had soups. In that case it would be fair to split it three ways. Maybe you even pay more! It really does depend on what everyone ordered.
But if you go out for a meal, set your boundaries at the start if you're worrying about money splitting. Just say I want to pay for what I have tonight. Something like that.
If this is something you struggle with Google how to be assertive. :)