I think £200 a month for 3 kids is reasonable BUT it's a big outgoing and it is not unreasonable to offer £30 a month per child, depending on DH's income NOT yours. It is NOT on you to financially support his kids, you got that? 
Don't be a mug.
My ex gives NOTHING, zip towards our 13 year old son. My DH has fully supported him- his stepson- since he was 7 and we have 3 kids in total. You don't have other kids to support and I reckon that the day you do, your DH may take a different stance... or not. I know it's hard OP and I do feel for you. I am in a different place but I do resent getting no support from my ex as our son is now a teenager and his needs are so much more expensive. I resent that he travels the world, spending loads of time in Thailand and Goa and can't even show an interest in his son's schooling- if he can ever remember where he goes. It sucks for my son, especially as he gets older. Deadbeat parents suck. But it is what it is.
There really is not much to be done. Your DH is doing right by his 3 kids. That's awesome and kudos to him. But there is nothing wrong with looking at your outgoings and earnings... if £200 a month is impacting your quality of life and say, you can't put petrol in your car for example or eat as well as you'd like, or pay your bills on time, then it is perfectly reasonable to cut back a bit. You can still financially support them within your means. There is nothing wrong with that. We all have to budget so sit down and do that.
Here's what you need to do to off-set your stress (and rightfully so... it's a tough schtick being a step-parent).
Get a contact order, seriously.
If you find that the time you are spending with the kids is too much and you are especially struggling during summer hols, having to take time off work to look after DH's kids, then you need to get a court order stipulating when they are with mum and when they are with dad- the non resident parent. But of course, sit down and talk to DH about this. You should not be missing work to look after kids who have a full-time stay at home mother.
I don't shoulder DH with my son's stuff. He has effectively raised him full-time from the age of 7 but I am mindful that he is a step-dad. I never expected him to take any time off or alter his schedule to accommodate my needs with my son. I would ask my EX to help out if I had to work. So make sure DH is not asking too much of you... is he off-loading parental responsibility on you that really the mum should be dealing with?
You don't have to be the good wife here and prove your worth by being 'the perfect mum' to his kids. Your role is to love- or just 'like'- his kids, be good to them, supportive of them, be there for them but understand that parental responsibility is on mum and dad, not you. You're not a doormat, baby sitter, or a the first port o' call if mum and dad are unable to accommodate their kids' needs. Be helpful yes, but if it's too much, say so.
You may think I am being tough, but I've lived it and learned it. I've had the hard talks with DH and I have angrily swallowed the fact that he doesn't love his stepson the way I do. He loves him... just not in that ideal way I would wish for him to. And why should I expect him to? He is good to him, treats him as an equal amongst his siblings, calls him our son, supports him fully, and does all the right things BUT DH really struggles with his role as stepdad. Why shouldn't he? I struggle being the bio parent, so as a stepmum, OP, you are entitled to sometimes really dislike the situation you're in.
Draw some lines in the sand. Budget and realistically look at DH's earnings and outgoings. Look at the areas where you feel you can realistically give as a stepmother and evaluate the areas where you are beginning to feel major resentment. Make the changes. You have to make sure this is working for you. You've taken on a big thing by marrying a guy with 3 kids. It will always have its headaches, for sure but it doesn't have to be a dark cloud over your marriage and shouldn't become this.