A baby mouse with it's eyes still closed sounds much too small for a 3 month old.
Hatchlings straight out of the egg can typically feed on hoppers already (the stage after their eyes open and they like to 'hop' around)
I bet if you got something larger, she would kill and eat it just fine.

Next time, just start feeding her straight in her enclosure. You don't need to bother with a separate feeding tank.
She won't develop cage aggression, which is a myth. And it's a lot less stressful on the snake.