What does ret do? Why is ret 0xC needed here? What if it was just ret and not ret 0xC or how about 0x4?
mov eax,[esp+10] // param3
mov ecx,[esp+0C] // param2
mov edx,[esp+08] // param1
push eax
push ecx
push edx
mov ecx,esi
call File.exe+333330
pop esi
ret 000C
In short, ret 00C cleans up the stack after call File.exe+333330. Before the call, you pushed three 4-byte values onto the stack (the contents of eax, ecx, and edx). 4 * 3 = 12 = 0xC (in hex). If you had ret without a value, it would have returned from your subroutine, but wouldn't have cleaned up the stack at all. If you had ret 4, it would have only cleaned up one of the values. ret 12 or ret 0xC takes care of all three.
See here for a similar question.