Events are said to be mutually exclusive if they can not occur at the same time, that is, the probability of those events occurring at the same time is zero.
In the question given above,
P (A) = .2
P (B) = .3
P (C) = .3
P (A U B U C) = .2 + .3 + .3 = .8
Therefore, P (A U B U C) = 0.8.