If you have 3 kids, you shouldn't have any problem getting exercise. LOL
Do you play "with" them? By that, I mean do you climb the monkey bars, push the merry-go-round, swing with them, run, catch, etc.? Do they like to walk anwhere? Put the baby in the stroller and take them all for a nature walk or something. Do you watch cartoons with them? Stand up and march while watching. Add some hand weights too. Maybe the kids will get involved and march with you and keep in shape. Or do some jumping jacks, whatever moves you.