A tercet poem does not have to have a specific rhyme scheme. It can go ABA, AAB, AAA, ABB, etc. It can even be completely unrhyming- for example, a haiku.
A tercet is a 3 lined poem with a rhyme scheme of (Aba-Ccc) example i once diedand came to lifeand then i bought a piewhen i bought the pieand i layed down to diei didnt die so i went to go and sigh