The problem is you are going to only be drinking 3 gallons of beer.
Nailed it.
What if it's beer he doesn't like?
Or perhaps using the batch to step up yeast to be harvested for another batch (I do this with lagers - 2 liquid vials of lager yeast for a 2.5 gallon batch, then use the yeast cake in the next 5 gallon batch and then that yeast cake in a 10 gallon batch).
Of course big beers would not be a good idea for this step process based on yeast stress, but there are times when it makes sense (especially like when you are making a beer that is really experimental).
But if you end up really liking the beer, then, yes, that is a problem only having 3 gallons...