If I played Death from Below and I chose a unit with 3 might or less, then if my opponent reacts to it and kills that unit, can I still play that spell from trash?

No, you cannot use the recast effect of Death from Below if its target is killed before resolution.

According to RiftJudge FAQ #9400, this interaction is ruled as follows:

Sequence of Events

1. Play: You play Death from Below, choosing a target unit at a battlefield.
2. Response: An opponent plays a reaction that kills that target unit before Death from Below resolves.
3. Resolution:
   • When Death from Below attempts to resolve, it checks its target. Because the target is no longer on the board, it is an illegal target (Rule 359.3.e.2).
   • The instruction to "Kill a unit" cannot be executed because the target is illegal (Rule 359.3.e.5).
   • The spell then attempts to check the target's Might to determine if you may play it from your trash. Because the target is no longer on the board, its Might is treated as "null" (Rule 359.3.e.12).
   • Since the condition "if it had 3 [S] or less" cannot be evaluated (it returns null), the "you may play this from your trash" effect does not trigger.

The spell resolves, but because the target is illegal, it fails to perform its primary instruction and cannot evaluate the condition required for the secondary effect.