@SW-User BlueVeins is correct. Look at it this way:
Your first roll doesn't matter; any number will do; probability = 1. 2nd roll needs to match the first; 1/6 probability. 3rd roll needs to match the first also, another 1/6. 1 X 1/6 X 1/6 = 1/36 answer.
Another way to look at it: consider all possible dice rolls where order of the rolls matters. 6 possibilities for each roll, so 6X6X6 = 216 possible rolls (order matters means a 3,4,5 is different from a 5,4,3). Anyway, among those 216 possible rolls, there are exactly six triples. 6/216 = 1/36 = answer.
Individually, you would need to roll one dice, at minimum, 6 times to possibly get a 3. That's the minimum probability.
So, that's 1/6, and each roll is a separate event. So, 6 events for each dice, and each dice is 1/6, you end up with: 1/6 * 1/6 * 1/6 * 6 = 1/36, or 36 different attempts are required to get the minimum probability.
The upper bound is limitless, because each roll of the dice, represents an individual event.