Hopper mice are too small for 3 month olds. You should at least be feeding them adult mice.

Mine eat anything you wave in front of them. I have never had a problem switching from rats to mice and back to rats. Live, F/T, whatever! They will eat it.

NOT eating ASFs isn't a bad thing. If anything its more convenient. ASFs are more of a last resort thing, unless you breed them as normal feeders.