When my youngest was a newborn, i babysat my husbands 2 month old cousin. We needed a little extra money and they needed the help. I also had a 2 year old, 7 and 8 year old. It was crazy, but we had a lot of fun. The living room was set up very much like a daycare. I started using my old double stroller and went for lots of walks. The hardest part was breastfeeding mine and bottle feeding the other baby at the same time! They always wanted to be on the same schedule...go figure! It was nice when i could get them all to nap on the same schedule.
You can do it :) It may be a little hectic, but once you have a routine it is not so bad.