Does every 3CNF propositional formula has an equivalent 2CNF propositional formula?
The single clause 3CNF:

$\varphi = a\lor b\lor c$

does not have an equivalent 2CNF. Suppose $\psi=\bigwedge\limits_{i=1}^m \psi_i$ is an equivalent 2CNF. Let $\psi_i$ be one of its clauses, and assume it involves the variables $a,b$. An assignment which sets $c=1$, but does not satisfy $\psi_i$, satisfies $\varphi$ but not $\psi$, hence they are not equivalent.