prog_char hello[] PROGMEM =
{
"HELLO1"
"HELLO2"
"HELLO3"
};
This is what "hello" contains: "HELLO1HELLO2HELLO3" (a single C string)
If that's what you want, then it is easy to locate "HELLO2" - it starts at offset 6, and is 6 characters long.
It might be easier for you to store them as separate strings.