I recently put two mob grinders in one room, one stacked on the other. If i put a third in there, should i put it in the top of the other? the side? or would it not affect the spawn rate with 3 in one room?
Don't think it matters, as long as you have enough clearance above and below each spawner. Skeletons, zombies, and creepers require two spaces above and below. Spiders require one space above and below.
As for spawn rates, as long as the spawners are for different mobs they will not affect each others' spawn rates.
For the sake of science you should build a spawn trap and record the drop rate, then build a second, third, fourth etc... and see how the drop rate changes.
On one of my old worlds i had about 8 poorly organized and designed mob traps within a 100 block radius. I could only sit by one at a time and all of them had very slow drop rates. I just started a new world and built 1 larger(but only like 2 levels) and the drop rate is phenomenal. I guess what im saying is, having multiple spawn traps seemed to affect my drop rate.
My design for the perfect multilevel trap: (edit: line up the two middle XXs,)
BBBBXXBBBB <---- spider spawn box
The addition of the spider spawn box means you need to make the drop chamber have a 1x1x2 entrance so spiders that dont die(most do) cant escape.