Hi,
I need a 4 inch cake to use sps, I have 3 inch pan.
What is the best way to proceed - try to bake to the top of 3 inch pan and fill or do 2 runs of 3/4 full 3 inch pan.
Thank you!

2 runs would be safer, then sandwich them with buttercream and skewer them.

I use 3 inch pans filled 3/4 and then tort them...giving me four layers of cake and a resulting 5 inch tall cake!...I just cut my SPS posts to fit. it's fine when I'm stacking three tiers but less than that it looks weird...so I end up only using three of the torted layers.

Thank yyou!
how many cake mixes does it take to fill 3 inch tall 12" round pan 3/4 full?

I don't use mixes so I really can't say but it takes about 11 - 12 cups of batter for the 3" pan. It depends upon the recipe I use because some batters rise higher than others.