when you say "owner build" do you mean hiring a Contractor, or actually building this yourself? do you have construction experience? if you do it yourself, it will probably take much longer, so that should go into your spreadsheet. also include time spent waiting for the drawings to be finished, for the loan paperwork to come through, and then time spent waiting for the permit to be approved.
not knowing your area (I'm in Florida), make sure you know what $85 psf gives you. here, that sounds like a DIY project and basic/minimal finishes. hiring a Contractor for a custom Triplex starts at $125-$150 and can go up with choice of finishes. (i'm an Architect, not a Contractor, and i estimate residential projects based on SF, too, but it's only a very rough estimate. a real Bid will be based on the different trades.) be aware that in most places, a triplex is considered "Commercial" so you will have to make sure the Contractors hold a Commercial Building license (different states have different licensing categories).
finally, there are site costs that aren't factored into a PSF cost - driveways, utilities, land clearing, finish landscaping, etc. so, having a partially complete or complete set of drawings and talking to several interested Contractors (not someone who throws you a price off the top of their head, but someone who actually takes a few weeks to review the drawings and have their subs review it, too) should be part of your evaluation, too. good luck!