Simple discrete math problem: A and B in 3CNF
How about $(A \lor A \lor A) \land (B \lor B \lor B)$?

If you need three different variables in each clause, you can use

$$
(A \lor x_1 \lor x_2) \land (B \lor x_1 \lor x_2) \land \\
(\lnot x_1 \lor y_1 \lor z_1) \land (\lnot x_1 \lor y_1 \lor \lnot z_1) \land
(\lnot x_1 \lor \lnot y_1 \lor z_1) \land (\lnot x_1 \lor \lnot y_1 \lor \lnot z_1) \land \\
(\lnot x_2 \lor y_2 \lor z_2) \land (\lnot x_2 \lor y_2 \lor \lnot z_2) \land
(\lnot x_2 \lor \lnot y_2 \lor z_2) \land (\lnot x_2 \lor \lnot y_2 \lor \lnot z_2).
$$