1. The temperature was below freezing.
  2. The temperature dropped below freezing.
  3. The temperature went down below freezing.
    Are all sentences OK?

I think it needs to be ‘below the freezing point/level’.

All the original sentences are absolutely fine.
1 and 2 would be used in preference over 3.
The suggested recast sounds stilted.


Yes, that is how it sounds.

Well, if you think so.

I know so.