With one friend (who doesn't drink alchol) i tend to pick up the bill maybe three times out of four. That seems fair to me.
With a couple of other friends, who all drink alchol, the bill is split three ways.
Same with another friend who drinks alcohol.
If you are really friends, it shouldn't be that difficult to have that conversation.
I used to regularly holiday with one friend. The deal was that each day, one of us would pay for lunch and dinner. The following day, the other would do the same. We agreed before hand that if either of us felt the split was unfair, we would discuss it.
I can see how it would be difficult to raise this issue noe, as it's been going on for some time.
I would say, if it's not causing you financial hardship, and they are good friends in other aspects of your life, I would just suck it up. Otherwise, maybe only go to one out of three invites. This may loose you friends, but so will building up unspoken resentment against them.