No. The convexity status depends on the values of the constants.
For example, at A = B = C = x = 1, the 2nd derivative is negative. But at A =2, B = C = x = 1, the 2nd derivative is positive.
Please read Why isn't CVX accepting my model? READ THIS FIRST! . You shouldn’t rely on forum readers to do your basic homework of verifying the convexity of your problem before asking for assistance in how to enter it into CVX.