Expanding once more, hopefully in even simpler terms, here are some possible results for 3d4:
1, 1, 1
1, 1, 2
1, 1, 2
1, 1, 4
1, 2, 1
etc.
If you completed this chart all the way out to 4, 4, 4 you would see that there are 64 different possibilities (whereas 1d12 only has 12). The explanation...