In the Pokedex entry for Gen IV to V, it says,
> It grows by feeding on gases released by garbage. Though very rare, triplets have been found.
Is it possible to ecounter a triplet Weezing?
No, there is no possibility of getting one. However, there is that third section of Weezing. This can possibly mean that that part sometimes becomes another face/body, and can possibly allude to an evolution in a later Generation like Gen 6.