If a unit survives Death from Below with Zhonya's Hourglass, can I still recast Death from Below?

Yes — you can still play Death from Below from your trash for [A] even though Zhonya's Hourglass stopped the unit from dying.

The recast isn't gated on the kill succeeding; it's gated on the chosen unit's Might ("if it had 3 might or less"). When a replacement effect negates the earlier action, the later linked instruction still executes unless it directly references the action being performed — the "if you do" wording, like Deathgrip, is what breaks in that case (Rule 359.3.e.14.b). Death from Below's condition only looks at the unit's Might, which Zhonya's doesn't touch, so the "do this" reflexive trigger still goes on the chain and you can play the card from trash for [A].

Two things to watch: the Might check uses the unit as it was when chosen, so a buffed 4 [M] unit means no recast; and the saved unit gets recalled to Base, so the second Death from Below needs a different unit at a battlefield — it can't target the one it just spared. Note also that Zhonya's is mandatory, so its controller can't decline the save to force a death.