I had a pair once that really liked live blackworms. (And wouldn't eat much else.) Theoretically, if they're hungry enough, you should be able to wean them to frozen. I try to wean my babies to mysis as soon as possible.
Sometimes it does take them awhile to settle in - and do remember that they can go longer than other fish between meals. As mentioned, they might still be stressed.
Oh, and knowing bangers, they'd just look at the guppies funny. (That is a really funny image...)
Good luck,
Danielle
PS. Why 3? Three is not a good banggai number. Keep an eye on them, and when 2 pair off, get that 3rd out of there ASAP. On the off chance that they're all female, you'll be ok. On the other hand, if they're all male, you'll eventually only have one.