What I did is I just got one of those small plastic dog/cat carriers and put a heating pad in there wrapped in a towel on the lowest heat setting, and put it in a room away from other animals and away from drafts etc

In the beginning when I first started bottle feeding kittens I was so paranoid, I did everything to the T, measured everything perfectly etc, but after awhile you just get the hang of it and it gets so much easier. Don't sweat it! If you don't wake up EVERY three hours, its OK, don't feel bad! I can't count the number of times I slept through the alarm, and the kittens all grew up to be happy and healthy little balls of fluff