Alphabetic Pattern Pyramid
Instead of having a static array of chars You could just use ASCII values from 0x41 to 0x5A. If You still insist having a static array, please consider marking it as a const.

Leaving Your int main() without any return value --- sign of the bad taste, to say the least. Consider having some proper error codes or at least return 0 on proper exit.

Your index of chars (letter2, letter3) can't be negative, so why would You use int instead of uint_* or size_t? Same applies to row and spaces variables.

In case of single char, putchar() is way faster then printf("%c", character).