Agree with what everyone else has said. I don't get on badly with my stepkids but after more than a decade I still don't feel completely relaxed in their company and I expect I never will.
On the other hand, it is an opportunity for them to see their dad and without a doubt he'll see it as "I only see them 2 days a week" as opposed to "I never feel relaxed at weekends". However nice they are three extra children every weekend is a big responsibility and if you're used to just one child I expect it feels overwhelming, more so if the house suddenly feel crammed as a result - not their fault - but practicalities like that can also add to the stress.
It is though, a very difficult subject to broach and you have to be extremely diplomatic and choose your words carefully. Not something I've ever totally mastered I have to say as no matter how tactful and positive I try to be at the end of the day DP goes into automatic defence mode if I dare say anything even slightly negative in relation to his kids. What I would do though, is as WestEast suggests and really encourage "special daddy time" between him and his kids on one day, so at least it's not quite so full on. This can potentially be sold as them really benefitting from some one-to-one attention - and that's not exactly a lie. Meanwhile, you can please yourself.
I don't know what the situation re: your own child is about them seeing their dad, but if she does, I don't think there'd be anything wrong in suggesting to your DP that once in a while you have a totally kid free weekend - people do, it's hardly a crime. That at least would give you something to look forward to and might make the full on contact weekends a bit more bearable. You can always suggest having them over for tea midweek if possible so to DP, it still seems as if you've taken him seeing his kids into consideration.
Another partial solution would be to use a babysitter on one of the nights. I do know, both from past personal experience, and from speaking to other SMs that some NRPs feel very reluctant to use babysitters when they "only" see their kids a limited time but again, parents using sitters is NOT out of the ordinary, and you shouldn't be made to feel neglectful or anything by suggesting it. After all, younger kids should be in bed at a reasonable time anyway so what difference does it make, really, if it's you sitting downstairs or a trusted sitter instead ? The point is, actually getting out of the house rather than feeling obliged to sit there can make a lot of difference to your mood.
And yeah, it must feel galling if the EX doesn't "work" (in paid employment). I totally agree raising kids is work in itself and yes, she has them every evening when it wouldn't be so easy for her to go out .... but realistically, all of us who are parents ourselves know that once all your kids are at school, and if you don't have a paid job, that the 5 or 6 hours you have free in the day won't be totally filled with housework, shopping or laundry and you will have a significant element of free time when you can relax. I appreciate that just like anyone, the ex wants adult free time at the weekends when she can completely relax if at all possible but if that's a valid desire for her then equally, your desire to enjoy similar leisure time should be just as valid, maybe more so as you don't have the responsibility of her children. The fly in the ointment is that if you want to spend that time with your partner, he might not agree and that's what needs to be carefully negotiated .... I personally don't see anything wrong in the typical EOW arrangement, maybe with midweek tea or meet-up as well, but obviously different people feel differently about that. I do think that at the very least one stepkid free weekend isn't too much to ask for every month. Our partners don't tend to think (IME) very hard about how these arrangements impact on anyone else (and I often think it's an arguably selfish decision to blank those considerations out) but what they tend to forget is that if you're feeling stressed out and under pressure you need an opportunity to recharge - and that helping you to achieve that would probably benefit everyone in the long run as you'd feel less resentful when you did see his kids.
P.S: with 3 extra kids, I really really hope that he's pulling his weight and then some, with all the extra work that entails. There's nothing more likely to induce resentment if you're expected to do all the drudgework (cooking, clearing up, beds, extra laundry etc) while he gets to do all the fun stuff.
