Hello all,
I'm quite new to the forum but I've been lurking around for a couple weeks. I'm also new to home brewing. I have 7-8 extract brews under my belt. This past Friday I made two batches. One was an all-grain (that's another post altogether) and one was an American Wheat extract kit...
Yes, a three gallon batch will be fine in a five gallon keg. You just need to bleed the oxygen out of the dead space. There are several posts on here regarding this topic. Here's one https://www.homebrewtalk.com/showthread.php?t=281610
Do a search to find others if you need more info.