Want a snappy scrambler for stumptown
With what you envision - A single cog in the rear and a triple crankset up front - you would still require a rear derailleur to take up the chain slack since the chainrings have such large difference in diameter. A chain tensioner would not cope with the slack created from shifting from a 42 to a 32 tooth chainring, for instance.

Since you'll need a rear derailleur for a multiple gear solution anyway, you might as well go with a cassette and single chainring.

For disc brakes you're limited to mechanical (like Avid BB7s, TRP Spyre) or the cable actuated hydraulic calipers (TRP HyRd) unless you spring for shimano Di2. 

The closest you'll get to a track/road frame that has the necessarry reinforcements and mounts for disc brakes is likely a cyclocross bike or some of the newer endurance road bikes (Trek Domane as an example).

You're unlikely to get something exactly as you envision, but with a few compromises (cyclocross with a cassette) you should be able to make things work.