Reject supervenience
Computationalism is only self-contradictory if all three premises are accepted. The first two postulates are integral to any computational theory of consciousness. The supervenience claim, however, can be rejected to prevent the inconsistency.
Argument anticipated by Tim Maudlin (1989).
The argument as anticipated by Maudlin
Maudlin writes:
"Since the computationalist cannot give up the sufficiency condition, and since it would tear the guts out of the notion that mental states arise from a complex computational structure to give up the necessity condition, our central focus must be the plausibility and force of the supervenience thesis. Here two avenues of approach are possible. Either the computationalist can argue that the thesis is fine but that, contrary to appearances, the physical activities with and without the machinery hooked up are not the same, or else he can try to renounce the thesis altogether. Let us consider these possibilities in turn" (T. Maudlin, 1989, p. 424-425).
Maudlin concludes:
"So the computationalist must renounce the supervenience thesis altogether" (T. Maudlin, 1989, p. 426).
References
Maudlin, Tim. 1989. Computation and consciousness. The Journal of Philosophy, vol. LXXXVI, no. 8, p. 407-432.