Scattered votes will have very little impact. The most impact we can have as voters in locked states is in concentrating on one party to boost for now. Party status and funding is based in part on percentage of the popular vote, so concentration is necessary for that visibility. While I wish it were more viable to vote in a manner I abhor less, I recognise it would be a better bet to shine light on the viability of third parties by bolstering one first.
The main thing is that you're entering a vote that isn't being counted in the Dem or Rep column.
If you don't vote then it doesn't count against the Dems or Reps at all.
Vote whichever 3rd party is easiest on your conscience, and you'll be voting against both parties that can't responsibly work together at the moment.