I am a 2.5-gallon brewer. Actually I went from 2.5 to 3 gallons, then recently back to 2.5 gallons again. Reason being is that I have so many experimental recipes that I want to try making, but I am not a real big drinker and don't give a ton of it away either. So right now, for instance, I have 3 different batches in fermenters, and a good ~4 cases of beer in my basement, and at the rate I drink it, it doesn't make any sense for me to be making any more than a couple of gallons at a time because it takes up so much space until I get around to drinking it.
Yeah, just split the batches any way you like. If you want to make 3-gallon batches, use 3/5 as much as you would use for 5 gallons. The math is even easier if you go to 2.5-gallon batches, then just split everything in half. Store most of the remaining ingredients in your refrigerator. Grain lasts for years with no refrigeration needed. Hops will last for years in the freezer but can also last for up to a year in the refrigerator if well sealed. Yeast has the shortest life. Dry yeast lasts for a couple years in your refrigerator, while liquid yeast only lasts for about 6 to 9 months. Always a good idea to make a yeast starter anyways.
Smaller batches allow you to experiment more. For guys like me, it is the only way to go.