Are you using dry or liquid yeast? Starters are not recommended with dry - just use two packages of fresh dry yeast.
If liquid, some people make starters in a growler or 1-gallon apple juice jug, etc. Use DME to make a THREE quart (liter) batch of ~1.040 wort. Pitch yeast on starter wort about five days in advance. It should ferm out in 36 hours, and when done, cool it to ~55F so the yeast settle out. When ready to pitch, decant the old wort and pitch just the slurry. Or buy THREE liquid packs/tubes.
EDIT: I entered 5.25 gallons of 1.085 ale at MrMalty.com's yeast calculator, and it said a 3L starter with two liquid packages, so this is clearly a case where using dry yeast would be easier if you are newer to starters and don't want the hassle. If your kit is a "yeast neutral" style you could use a dry yeast. Because of the high OG of 1.085, I'd recommend rehydrating the yeast per mfr's instructions to maximize viability.