This was the logic: The 2 last conditons of your formula are:
IF A3 < 0 and A3 - C3 < 0 then A3 - C3 else 0
IF A3 > 0 and A3 - C3 > 0 then A3 - C3 else 0
They are very similar. You can write them in a way that they are almost the same:
IF A3 < 0 and (-1)*(A3 - C3) > 0 then A3 - C3 else 0
IF A3 > 0 and (+1)*A3 - C3 > 0 then A3 - C3 else 0
The number multiplying (A3-C3) in the condition in both cases is the sign of A3. So the value of the expression is, for both A3<0 and A3 >0:
(SIGN(A3)*(A3-C3)>0) * (A3-C3)
As you know, excel converts true to 1 and false to 0 in an arithmetic expressions. So if the condition is met its value is 1 and you have the result (A3-C3) . If the condition is not met, its value is 0 and you have the result 0.
Now we look at the third condition: If A3 = 0 then result 0. Usually to include that condition you would just have to multiply by (A3<>0).
However in this case this would be redundant. In fact if A3=0 then SIGN(A3)=0 then the condition is false and you are multiplying 0*(A3-C3) which is already 0.
So the last expression encompasses all three conditions.