FILE *MyFile, *OUT;
char temp;
Symbols[3] = {'a', 'b', 'c'};
......

MyFile = fopen("1.txt", "r");
OUT = fopen("2.txt", "wb");
while((temp=fgetc(MyFile))!=EOF)
{
  for(i=0; i<3; i++)
    if(temp == Symbols[i])
	fwrite((const char*)&a[i], 1, sizeof(a[i]), OUT);
}