Assume that "batting average" is his probability of "batting".
So, for Jerrit to get exactly 1 hit, he has 3 ways, , to do so:
get a hit in 1st try (and NOT in 2nd and 3rd) OR
in 2nd try (and NOT in 1st and 3rd) OR
in 3rd try (and NOT in 1st and 2nd).
Put this into equation:
If your question of "getting only 1 hit" means "at most 1 hit", add probability of 0 hits: .