I think the "you can't pay for everything with the stipend" kind of diminishes if you are talking about multiple children. That is because your expenses (such as rent or gas)arent necessarily doubled for two children, or three children. Its absolutely fine to use some of the stipend for rent...that is the child's "room and board"...part of the stipend goes for their share of all household expenses (electric, heat, water, gas to appointments and activities etc). Frankly i dont have a problem with a FP "profiting" if there is any to be had...you ARE actually working. It can at times be hard work, and i'm not even necessarily talking about the care of the children (which may be easy or may be the hardest thing you'll ever do, depending on that child's needs)...but you will be having monthly (or more often) social worker visits in your home, if there is parental visitation in place, you may have to transport to that (i had to drive 80 miles round trip once a week for about 2.5 hours plus driving time...it really screwed up our Thursdays let me tell you), plus you have to take training on a yearly basis, you have to go through recertification visits, there may be tons of paperwork. None of that is necessarily difficult, but sometimes in can be a PIA and very intrusive in your life.
I'm a fairly low income mom and had to move to a larger place in order to have my foster son's sister placed with us (TPR had already occurred and i knew i'd be adopting her), my rent increased by almost $300 when i moved. So, yes, i was counting on the additional foster money to cover that. The thing is, you need to be sure that if something were to happen and those kids moved, that you wouldnt be in a horrible bind. I knew that if for some reason my daughter *wasnt* placed with me, we could still afford this place, but we might have to really tighten the belt. Basically...have a back up plan.
You may find that your foster kids qualify for free or reduced price stuff, such as summer camp (our worker called and said my daughter might get to go to camp this summer due to being on some list, but we will pass because i want to be free from that agency but thats another story!)...they will get free school lunch, medicaid, they may qualify for something like after school program or daycamp in the summer for free or reduced price depending on how your state works. There are often clothing allowences separate from the monthly stipend, depending on your state. You must live in a great state (or a very expensive one!) for $900/mo to be the basic stipend. Here in MI its just over $400/mo (kids over 13 get more, i think around $500/mo), the maximum you can get without super high level rare approval is i believe 800-900/mo, although my daughters former FM claimed she was getting $1600/mo for doing therapeutic foster care for teens, so maybe there is more to be had in specialized programs. (There is basic level which is 14.-something a day i think, then Level 1 which adds an additional 5/day, Level 2 adds 10/day. Level 3 would be extreme needs and adds 15/day.)
As for what i pay...as i said, part of my rent and other expenses...but for my little son who is 3, its hard to say, he gets toys like my adopted son, clothes, meals out, the occasional visit to a local play place that has an entrance fee, that sort of thing. His major expense is probably diapers. My daughter is 9, and she gets all of those things too, plus an allowance and things like cd's and whatnot but usually only on special occasions. We arent into scheduling a million activities, but she does take piano every week (70/mo) and i imagine when the boys are a bit older they will start as well. The thing with having multiple kids is everything is multiplied...going out to eat starts becoming cost prohibitive. Or when it was just my first adopted son, if he wanted a toy in the store i'd just get it. What's five or even ten bucks? But with three (well four, but my oldest is beyond begging for toys ;) ) suddenly i'm looking at 30 or 40 dollars and thats just not something i can casually spend like that. Or going to the movies, with one its no big deal, but then if you are taking FOUR kids out...suddenly you are spending 50 bucks or more. But there are ways around that...like, when we eat out its usually at a buffet where the little ones are free or just a couple bucks, and i may use a coupon for a percentage off the bill...or you could go to the "cheap" movies. Basically the same tools any large family without a lot of money would use.
Beyond looking at the money aspect, i'd encourage you to look at the impact of adding three children to your presumably happy home, are these kids with issues? You know the kids, but do you know if there are behavioral concerns? Acting out? How do they fit in, age wise, with your son? How would he feel about a suddenly large family? Do you have support to help with things like childcare if needed (family or friend support, i mean)...is there a plan of reunification with the birthfamily and can you support that? Of course you probably have thought about all these issues but i'm just throwing that out there!
Sorry this was so long!