If I am not mistaken, It will probably crash the program.
1.)The space for the pointer p was not allocated from the system. This means it does not belong to you.
2.)Declaring main with (or without void), is undefined behavior, you should use int main().
You are incrementing the value of a char, so you are changing the encoded value. For example a may be represented as 100 for computers b as 101 and c as 102. So if you increment 3 times you may get c (it depends if they are in order. 103 for example may represent something completely different). You can express it as ++(++(++p)). If p is a, then this statement should be c. But if you add 3 to a, the compiler will probably convert a to integer so you will get number outputted to the screen.
You can also use typecasting (variable type). For example if you try to output (char)101, you should get e (or it may be another character).