I make a lot of 'half' batches and now that I'm kegging (thank you Santa!), I regularly keg 3 gallons in a 5 gallon corny. I've done 5 so far this year.
A couple of things I've noticed: 1) It is really easy to overcarb 3 gallons, especially if you're force carbing. I've had better success with the set and forget method with the occasional purge; 2) Dry hopping in the keg can be tricky. If you're not careful, you'll drain the keg lower than a standard hop bag usually sits. If you do dry hop in the keg, be sure to account for the lower liquid level; and 3) 3 gallon kegs go really, really fast. At least at my house.