Hi Jessica,
I totally relate to the 3 kids under 4 situation you're in. My DD was 2 when my twins were born (Now they're 5 1/2 and 3 1/2). Don't stess yourself out about your workouts postpartum. You'll know if you're doing too much too soon, if you just listen to your body.
My doc was pretty happy with my ability to move at my first checkup and told me to just keep paying attention to how I felt. We do know ourselves and our bodies, don't you agree?
If you can't go out for a walk with the stroller (as I could not, either), Leslie Sansone is a fine workout to do. What I found the most helpful was doing what I knew I would look forward to - find the workout you enjoy and do what you can. I would sometimes pop in a Cathe, using one of the shorter premixes and just do what I could of it. Yes, I got interrupted when babies woke up or needed fed/changed - BUT, I felt better for making the attempt. And I didn't freak if I missed some workouts.
I have a saying that I read alot during that time, it's more about housework, but it applies to all the stuff we try to do. "Cleaning and scrubbing can wait till tomorrow, for babies grow up, we've learned to our sorrow. So quiet down cobwebs. Dust, go to sleep. I'm rocking my baby, and babies don't keep."
I know you'll do just fine!
Tricia