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To be brief - I am a rising 3L @ CCNMVP, recieved very low 1L marks due to legitimate extenuating circumstances. Above median 2L and doing my summer at a small regional firm (no chance of full time offer)

I want to do biglaw but my resume is unspectacular and I am an average interview on my best day - is there any utility in spending time trying firms or would that be futile?

Probably not worth spending too much time on it. I'd focus a few letters on the firms that generally hire 3Ls and hire 3Ls extremely late. I know Latham is one, I don't really know what other firms though.