Dear M.,
I add my voice for you having a separate account.
Deb put it wonderfully:
in the most friendly manner, to sit down and look through all the payments, deciding, who can and will contribute what.
I am a teacher too, it's A LOT OF WORK, and A LOT OF TIME: this work does not end when you close thedoor to the classroom, you think of our class in the evening, see the troublemakers in your dream, thinking ho to help them to become better people, check their homework, and prepare tests for them... this is one work of dedication.
Now,you put your heart in your work, AND in your family. This is GIVING. In order for you to be able to contribute as much as you do, you need a part of RECEIVING to be in balance, also. You cannot just EXhale all the time, you know, it's a pattern: you need to INhale at times also, otherwise, you will be out of breath!
I would suggest: there are three adults who now care for your husband's kids. He loves them, you love them, I'd wish to say, their mom loves them. All every one of yo adults also have LIFE TO DO in this world where $ is an inevitable part of it.
HE NEEDS TO BE HONEST and tel you exactly what their mom's situation is, and what his situation is financially. You need to make a chart, or a graph, to put out who can contribute how much, and stick to it. I mean, you could say: "i help 1/3 of the kids' expenses" but as you are FRIENDS in the first place, then it's good to HONESTLY converse about what the heck is going on, and how to set it right. He already knows you don't mind helping, but he needs to also know you're not a pack horse, to ride you all along. You definitely need your own share; a teacher needs to be happy, in order to be able to make kids happy, and help them to become happy people!!! A crying miserable teacher cannot contribute so much to kids in class: talk to him about it, he does not realize at all what it is to be a teacher, btw! Those not teachers go to work, do their ork, come home, and they leave all their job-thoughts there (most jobs are like this), but not yours, (and some other jobs). Then, you need
some decent clothes, you cannot run around in one skirt for a year, and then, you need some extra money to even go and get some Christmas presents to your very own husband and his=your three kids - sit down and talk to him in a very friendly way of real two freinds, not accusing him, but putting it out in reality: ti show him that in your request, you are not selfish at all, but you need common sense, and MUTUAL care in this relationship!!!
Good luck to you, M., and stick to your friendly guns!