How to house my seramas...
I have never kept seramas, but I do have bantams so I hope I can help some.
Seramas are more delicate to the cold, they don't do well in temperatures nearing freezing and shouldn't be kept in temperatures below freezing most certainly. However, they will love time outside while it is warm and it will encourage activity and natural behaviors. Make sure there are no small gaps for them to slip out of, as they will certainly do so with the chance. Provide plenty of shelter from any wind or rain and nice, dry bedding to snuggle into during the night.
This should help you further: http://www.backyardchickens.com/t/583308/how-do-i-keep-my-little-seramas-warm-in-the-winter
Best of luck!
The only reason that being in the garage would be bad (unless their enclosure is too small) would be because of any strong smelling chemicals or gasoline, which are very dangerous for any birds delicate lungs.
If you are worried about them being bored indoors you can offer fun perches, dust boxes, dirt to dig in, grass clippings, hidden treats, melon rinds, and other fun things for them to dig into, jump on, and hide under. I've found chickens in smaller enclosures love having something to do.
They are maybe 3 months old...I was told by the guy I got them from they didn't need any treats or anything til 5 months, is that true?