Can a math person help out on a problem?

you have 3 dices

1 has 6 sides
1 has 12 sides
1 has 20 sides

what are the chances you get the same number with all three of them?
give that you can only get the same number with all 3 with 6 numbers (1-6)
how do you calculate the chances and probabilities? i mean the math sequence of calculating this?