It usually depends on the motherboard. Nowadays many (but not all) motherboards can accommodate any number of modules in any slot. But some motherboards have restrictions.
I remember my first Windows machine back in the 90:s, it had 4 slots. Slot 1 had to be populated, slot 2 could be empty or the same size as slot 1, slots 3 and 4 could be empty or both the same size. I remember having 4+0+4+4 MB in that machine at delivery.
So in most cases you should be able to put 3x4 GB, but you really need to read the manual for the motherboard to be sure.