Find $a, b, c$ minimizing MSE

Suppose you are given a dummy classifier. It looks like this: $$ y(x) = \begin{cases} a \text{ if } x = c \\ b \text{ else } \end{cases} $$

Given some data set $\{(y_1, x_1), \dots (y_n, x_n)\}$ how to estimate $a, b, c$ such that the MSE would be minimal?