Let's get the number of SUCCESSFUL ways:

There are only 6 successful ways they can come up:
All 1's, all 2's,..., all 6's.

Now let's get the number of POSSIBLE ways:

There are 6 possible ways the first die can come up
TIMES
6 possible ways the second die can come up
TIMES
6 possible ways the third die can come up.

So that's 6×6×6 = 6³ = 216 ways.

So the probability is 6 successful ways out of 216
possible ways, or 6 out of 216 or 6/216 which reduces
to 1/36.

Edwin