If he hits 8 out of 10 shots on average, that means that his probability of success is 8/10, or 4/5. If we assume that this probability remains that same with each shot, i.e., that the outcome of each shot is independent of any prior shots, then this will be a binomial random variable.
The probability mass function for a binomial random variable is
P(X = x) = (nCx) · px · (1 - p)n - x, where x is the number of successes, and n is the number of attempts.
So we can now plug in all of our information:
P(X = 3) = (4C3) · (0.8)3 · (0.2)4 - 3 = 0.4096.