Hi K. - I was in the same boat as you about a year ago - mom to a 3 and 5 year old, expecting our 3rd. The good news is that your kids are old enough to play well by themselves and possibly even be helpful. But I don't know if there is anything you can do to make it "smooth" - it took us a good two months to even feel comfortable taking all 3 kids out of the house.
Fortunately, my son, who is now 15 months, was the most mellow, laid back baby who slept well and ate well - which I attribute to the fact that I was so busy with the other kids and the business I was running, he had to learn how to sooth himself much earlier in life. Breastfeeding was great for me, though - it forced me to take the time to sit down, rest, feed the baby and have some time with him.
Well - guess what - now I'm expecting #4 in August - a HUGE surprise. Any advice on how to make that go smoothly? LOL!