How do we explain the result of the expression (++x)+(++x)+(++x)?

We explain it by expecting undefined behaviour rather than any particular result. As the expression attempts to modify x multiple times without an intervening sequence point its behaviour is undefined.

As others have said, the C and C++ standards do not define the behaviour that this will produce.

But for those people who don't see why the standards would do such a thing, let's go through a "real world" example:

1 * 2 + 3 + 4 * 5

There's nothing wrong with calculating 1 * 2 + 3 before we calculate 4*5. Just because multiplication has a higher precedence than addition doesn't mean we need to perform all multiplication in the expression before doing any addition. In fact there are many different orders you validly could perform your calculations.

Where evaluations have side effects, different evaluation orders can affect the result. If the standard does not define the behaviour, do not rely on it.