Three chips aren't working right now....I suppose taking it apart and re-doing the thermal paste is an option?
No, that won't help for anything other than thermal conveyance. I've had chips show as 'x' on my S3, I usually just power off the machine for a few minutes and fire it up again and they have always come back.
I'll try that out, I just purchased them used. Most are ok, there are a couple miners that show a couple x's a piece, nothing major I assume. How do I know if the chips are over heating? They all have temps of around 39C.