3. The equation $\log _{2} x=3 \cos x$ has $(\quad)$ solutions.
(A) 1
(B) 2
(C) 3
(D) 4
3.C.

Draw the graphs of the functions $y=\log _{2} x$ and $y=3 \cos x$, and study their intersection points. It can be observed that there are 3 solutions in total.