1/8
1) Considering all the possible numbers when we roll a die, in this case, 8 sided die:
1 , 2, 3, 4, 5, 6, 7, 8
2) Since we'll roll it once, we have 1 possibility over 8, therefore we can write:
3) Hence, we have 1/8 as the probability of getting a 3 in this dice, considering that all numbers are equally possible.