If rob flips 3 coins what is the chance that they come up all tails

1/8. 1/2*1/2*1/2 = 1/8. Each 1/2 represents the probability of getting tails on each flip. We multiply three of them because there are three flips.

One way you could solve this would be to write out all the possible ways the 3 coins could land. There are 8 different arrangements. 

H-H-H
H-H-T
H-T-H
H-T-T
T-T-T
T-T-H
T-H-H

You can see that there is 1 possible way that the coins give you the result you asked for; three tails. So that's 1 out of 8.. 1/8 = 0.125 = 12.5%