This is a major edit of a question which had number of ambiguities. Hopefully, the edits will clear the ambiguities.
So, A, B, and C were co-tenants on a lease in an apartment in California and A, B, and C used to pay the rent together to a landlord. And co-tenant A wanted to move out of the place. So A convinced B and C that if as co-tenants A,B, and C sign the new lease identical to the original without A's name on it (effectively "removing" A from the lease), A would in exchange, provide postdated checks to the landlord for the next three months of "A's portion" of the rent, plus the lease change agreement fee. So B and C signed the new lease without A on it.
Note that here, A's agreement to pay "A's portion" (which technically isn't spelled out in the lease) of the rent was only communicated to B and C, and not the landlord. In fact, for the lease change, the landlord did not inquire nor care about who will pay how much - just that the rent for the place were to be paid by somebody (including possibly those outside the lease).
Soon after A moved out, B and C found out that A in fact, asked the landlord to give A back the postdated checks, and B and C are the only ones obligated to pay the upcoming rent.
So in summary, what's remaining are 1) the new lease which is identical but without A on it, and 2) rental obligations of B and C (which are now increased), and 3) some traces of A saying that A will pay to the landlord "A's portion" of the rent after the lease change.
From some of the answers to the original question, there seem to be contract between A and (B and C) for paying "A's portion" of the rent, and another contract, the new lease between (B and C) and the landlord.
What legal actions do B and C have against A? And which actions are likely to succeed?