Having a look at the FCOM in Abnormal and Emergency Proc's, the status page for dual ADR failure only shows ADR 1+3 as the only combination that requires a manual gear extension (with subsequent no retraction). While I have always contemplated turning off 2 and 3 in the event of that failure popping up, I have to say I do like iggy's idea of turning 1 and 2 off so we can switch #3 to either side as required. Something to ponder about.
As a side note, turning off 1+2 apparently would leave you without "ATC/XPDR". I imagine it is only the data provided by the ADR's, not the xpdr signal as such?