EDIT: Fixed errors on the piecewise.
From Luke:
p(getting HIV) = 1 - p(not getting HIV)
= 1 - (1 - 0.35)^3
= 0.725375 or 72.5%
From Charlie:
= 1 - (1 - 0.6)^2
= 0.84 or 84%
From Matt:
= 1 - (1 - 0.25)^5
= 0.762695 or 76.3%
For the piecewise function, first assume that the problem asks...