It sounds like you'll be getting enough exercise just from taking care of three little ones. The challenge is going to be making time to take care of yourself and eat right. You really want to be getting 80-100 grams of protein per day, a well balanced diet, and lots of water. Also, stay away from the urge to just eat junk (fast foods, too many additives and colorings, carby sweets, highly processed foods, etc) all the time because it's quick. Here's a couple resources on healthy pregnancy diet:
Ideally, you should be getting your heart rate up (but not over 160) for 20 minutes a day. You could go to the gym if you have childcare, or walk with the kids in a stroller.
You might want to arrange to have good help. Maybe you can hire a teenage mother's helper. I've got one for the summer for 4 hours a day for $100/week. She's 14 years old.
Also, after the birth, you will burn calories by breastfeeding, plus regular exercise, and good diet.