Is the Keb Three Season warm enough for winter?

No. It is a three-season bag built for spring, summer and fall. For freezing winter nights, look for a bag rated well below this one.

Down or synthetic for this bag?

This one uses down, which is lighter and packs smaller than synthetic fill. The trade-off is that it loses warmth if it gets wet and needs more careful care.

Does the mummy shape fit side sleepers?

Mummy bags are narrower at the feet and shoulders, so side sleepers often feel restricted. If you move a lot at night, a wider cut may suit you better.

How should I care for a down sleeping bag?

Wash it rarely, with a down-specific cleaner, and dry it thoroughly with dryer balls. Store it loose in a cotton sack, not compressed in its stuff sack.

Is it good for backpacking?

Yes. The down fill and mummy shape keep weight and packed size down, which is what backpackers want. It is less suited to wet, humid trips where synthetic fill is safer.