You can but be careful about taking too much from any single plant.

no problem, just make sure your mother plant is very healthy

I have thought about doing the same thing, but I have more than enough petunias at this point and am out of space. You definitely want to at least pinch back the cutting, so they bush out. Three weeks seems a little young to do more than snip off one piece, but if you do that, in three more weeks it may be ready to give you several cutting from each.