Britt, I would say that you would need to let the kittens be your guide. At 3 weeks old, you may be able to begin inroducing them to a really soupy, mushy, canned kitten food now - I use Nutro Natural Choice in the small cans. It is the very soft, pate-style food and I mix it with a little warm water to make it more of a runny paste. You can try this along with bottling to see if they will take it.
At 3 weeks, I would bottle them about every 4 to 6 hours, more often if they seem to need it. One sure sign of needing to eat is a wrinkly little belly. Kittens should have a nice, plump, rounded belly with the skin tight across the belly ... if you see little wrinkles, feed them more often.
As for stopping the stimulation ... I normally introduce the litter at the same time I introduce regular food. My momma cat normally shows them what to do, but in your case, not having the momma cat there might be more of a challenge. At this point, I would suggest that you continue stimulating after each meal for now. Make up a litter box for them ( a low-sided baking dish is perfect for this) and after you have stimulated, take a little of the elimination and place it in the litter. After you feed them the next time, place them in the box one at a time and begin the motions of stimulating. Once elimination begins, they are in the litter and instinct should take over.
Best of luck,