Bayes Theorem

How would you use bayes theorem to calculate the probabilty of archer B hitting a target, given that one of them has.

Probabilities for the 3 archers:

A=0.6
B=0.3
C=0.5

without using bayes theorem i can get an estimate by doing 0.3 divided by the total. Which comes out at about 0.21 (done from memory)