Just take it one day at a time - if you miss a day, don't let that be an excuse to be a couch potato (or too busy) the next.
Schedule your workout time, and make it a top priority - enlist your family's help to figure out how to get this time for you (and for them, because a healthy mama is a happy mama).
Try to build more exercise into your day. Park far away from the grocery store and push the cart all the way back when you're done with it. Get a backpack and carry your smallest child rather than using a stroller. Walk up and down stairs whenever you can. Try to take an activity break every couple of hours - walk outside for 15 minutes, do 50 crunches, little bits of exercise really add up.
And take a good look at your diet. Gaining 3 to 5 pounds in a week means eating 10,800 to 18,000 calories more than you're burning, which is a heck of a lot. Don't let weeks you work out be an excuse to eat unhealthy foods or eat more healthy foods than you should - try keeping a food journal for a couple of weeks and identify every last thing you eat (and how much).
HTH! You can do it! Just take it in baby steps.