Does Hextech Ray allow you to redistribute any excess damage to remaining units on a battlefield?

No — Hextech Ray has one target and deals all 3 to it; there's no split and no carry-over. It reads "Deal 3 to a unit at a battlefield," so it targets a single unit and that unit gets the full 3 even if it only has 1 or 2 Might. Excess damage above lethal just sits marked on that unit until it's killed in the next cleanup (rule 142.4.b).

The "lethal in full before moving on" restriction you're thinking of (Rule 467.2.c.3) only governs combat damage assignment — it isn't a license to redirect spell damage either way. Spells that do split damage (e.g. Alpha Strike, Scorched Earth) say so explicitly and choose their targets at finalization; Hextech Ray never does.