You say you can keep 3 mice, but not 3 rats? Can I ask why?
IMHO, rats are better pets than mice. You can get them out and let them run around without worrying about them getting loose and never coming back. Mice are not as relaxed (in my experience) than rats. Rats can become your best friend in a second; don't get me wrong, I know that mice are some people's favourite pets. Also, they're no harder to take care of. You still have vet bills, food, cage cleanings, playtime, etc.
And yes, those do look like baby rats. Cuties, too bad I can't take them.
Wild rats are different than domestic rats, yes, but I don't know about being raised from almost birth. My guess would be they'd act somewhat the same but maybe a little different instincts. I know of people who have taken in wild rats. I also know of someone who took in a wild mouse and housed them with domestic mice. So, maybe it's the same with rats - they can still live together, no problem.
I don't know how rat rescues are going to feel about someone raising rats and then just dropping them off there. You might want to consider finding them a forever home instead of a shelter for them.
You said they were found under your fiance's motorcycle, right? Were there any other rats in site? Usually (some momma rats aren't like this at all!) rats have more than 3 babies. Unless she realized she couldn't nurse all the babies and left 3 purposely behind, I'm guessing she would've come back for the remaining three. Maybe I'm just weird, but I wouldn't have "bothered" them for a while. Just checking on them without invading. I would've looked for a nest or the sign of a momma rat. But yeh, maybe I'm just weird.
Good luck with whatever you do.
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