You can make a 2.5 gallon batch in your 3 gal carboy. No secondary required.
I'm not an expert on cider but I make 2.5 gallon batches pretty often. My recipe is as follows:
24 oz. apple juice concentrate
~ 2.25 gallons apple juice
Nottingham - I prefer notty to champagne or wine yeasts, I think it clears better
Optional Additives:
Pectic Enzyme which helps with clarity - 1.25 tsp
Wine Tannin which gives a little bit more body to the cider, especially if you're leaving it dry - 1.25 tsp
Malic Acid, this is the primary acid found in fresh apples. It will give the cider a bit more of a crisp, tart flavor more like that of a fresh apple. I usually just add a bit at a time and taste until I like the flavor before pitching.
Ferment 2 weeks at ale temp then cold crash if you can for 24-48 hours.
Post fermentation you can bottle still and have a decent dry cider, or you back sweeten with artificial sweetener, prime, and bottle, though I've never done this personally.
What I do is back sweeten with another 12-24 oz of apple juice concentrate, bottle, and then pasteurize on the stove. This produces a sweet, carbonated cider like strongbow or even angry orchard. This is really risky though, especially with as much sugar as I use. I have had multiple bottles explode on me while doing this, so I can't really recommend it for safety reasons. Others seem to pasteurize without much issue, but they're probably not sweetening anywhere near the level that I do.
