Not a stupid question.

Some people get very involved in knowing all the right gear combinations, which one to use in which situation, power ratios, etc. Very technical, and not necessary.

I like to keep it simple.
I also have 3 front rings and 7 rear gears (cogs). I usually start in 3-6 or 3-7 ('cause that is the gearing I usually end with) and shift to a gear that is comfortable to pedal at 80 to 90 rpm (cadance or pedal turns). If I feel too much resistance, I shift the rear one gear easier. Too little resistance, I shift one gear harder. When I'm in the middle of the gears, usually 4 or 5, I shift to the middle ring in front. If I reach the bottom of the gears and still have too much resistance on a hill, I shift to the small ring and use the 1-1 combination. As the hill gets easier I reverse the process - 1-1 to 1-2 to 1-3 to 2-3 to 2-4 and so on.

I usually don't even look at the numbers any more. I just go with what feels right. Going from highest to lowest gear is done in just 8 shifts - 6 shifts for the gears and 2 for the rings.

Shifting 2 or 3 gears at a time is no problem. Trickier is shifting the front and rear at the same time. Do it wrong and your chain falls off.