Decrementing inside of an if statement C++

I am confused as to how to have equivalent logic without having the the decrement inside of the if statement.

if(A && !B && !(C--))

I thought that this is equivalent to:

if(A && !B && !(C))


In the first example, C is decremented if the first two conditions are true.

In the second example, C is decremented if all three conditions are true.

The difference is that if !C is not true (the third condition is false), the first example will decrement C while the second will not.