You should have trouble predicting it, because it is in fact unpredictable. This is what's known as undefined behavior. Modifying a value more than once in an expression is legal, but undefined.
The line
b = (++a) + (a++) + (a++);
invokes undefined behavior because you're attempting to modify the value of a more than once between sequence points. Any result is considered "correct".
Should be 7. I read it as increment a by 1, then add a to a, then increment a, then add a, then increment a. This comes out 2 + 2 + 3 = 7.