I don't advocate the use of this macro, but if you do use it you should really wrap the parameters and the whole expression in parentheses to avoid precedence issues:

  #define TERN(c, a, b) ((c) ? (a) : (b))

That isn't more readable. It's non-standard. It also looks like a function call, but doesn't behave like one (in a function call, a, b, and c would all be evaluated). It's going to confuse other C programmers.

As Liquid_Fire points out, it's also probably incorrect.

You may wish to look at the Bourne shell source for other macros to make C readable.