The answer is 216 because we will compute 6^3 = 6*6*6 = 36*6 = 216
If we had 2 dice rolled out, then it would be 6^2 = 6*6 = 36. For each die added, we multiply another 6 because the die has 6 sides.
Visually, 2 dice form a 2D table of 36 possible outcomes. Three dice form a 3D cube of possible outcomes (a 6 by 6 by 6 cube with volume 216). You can think of it like a rubix cube. Each sub-block would house a possible combo such as {2,1,7} meaning that you rolled a '2' on the first die, a '1' on the second die, and a '7' on the third die.
Note: rolling a single die 3 times will produce the same sample space as rolling three separate dice 1 time.