No, a non-zero $\beta_3$ does not imply $A$ and $B$ are correlated. It implies $y$ is correlated with $AB$.

## Trivial example:
Imagine we have data on visits by people to a gas station.

 - Let $A$ be the volume of someone's gas tank in gallons.
 - Let $B$ be the price of gas at the time of the visit.
 - Let $y$ be the spending on gas this visit.

$A \cdot B$ is how much it would cost to fill the person's gas tank. $AB$ is almost certainly correlated with $y$, the spending on gas this visit.

A positive $\beta_3$ in this trivial example does not imply that the size of someone's gas tank is correlated with the price of gas. A positive $\beta_3$ would mean that spending $y$ is positive related to the carrying capacity of someone's gas tank measured in dollars (i.e. $AB$).