I'll apologise in advance OP, because this is going to come across really harsh.
"I'd do anything for them"
Not quite true. The one thing you apparently will not do is make them stand on their own two feet. Sometimes it's hard, but our job as parents is to prepare them to face the world independently; not to wrap them in cotton wool.
"They didn't graduate until 24 so I couldn't cut them off earlier."
24? So they did a Masters/PhD? Further study was a choice on their part, begun when they were already adult, and maybe you should have allowed them to make the choice on their own (rather than influencing their choice by financing one option). Maybe they should have considered they couldn't afford to do it, or needed to do it part-time and work to finance it.
I'm a bit confused re your finances.
"My ex won't help the kids financially on the grounds that he is paying me maintenance which he says should cover me and the 3 kids. The kids agree." Do you agree? Is he paying enough?
"So I am using maintenance/savings and/or my pension to help them."
So if part of it is maintenance and part of it is your savings/pension, then your children are being financed by both of you (assuming he is paying enough maintenance)?
"I love them all dearly but they don't see what I do as coming from me, their Dad gets the credit."
All the credit? Or just some of the credit? He's entitled to some of the credit if his maintenance also maintains them. (Again, only if the amount is actually enough.)
"it's the kids attitude I find so hard."
Again, not quite sure of what that attitude is. Is it that they credit your ex for the financial help and not you, or is it the attitude that they should be financially helped despite being adults?
"Now the older ones pay ridiculous high rent and are completely cleaned out by rent and travel costs."
Then they need to cut their coat according to their cloth. Flatshare, move, whatever they need to do AS ADULTS.
"Not sure what I'd achieve by cutting them off".
Their independence. And the end of your martyrdom. Sorry, but you really shouldn't be financing them at their ages. It's not healthy for your relationships with them.