Ahh.. well given that you want triple chainrings, if the geometry is likely to work for you, I'd recommend having a Soma frame built up. Depending on how it's done it can be reasonable in price or pretty spendy - but you'd get to pick absolutely everything you want on it

Velo-orange also sells a mixte frame, with slightly different geometry (and only in blue). http://store.velo-orange.com/index.p.../mixte-72.html

You could also potentially buy a vintage mixte, strip it down (you could even have it painted at that point - I did that with my kid's 24" mixte), have the frame respaced and then build it back up however you would like - I actually did this on my Takara, the only thing on my "wish list" is to replace the original center-pull brakes, but I just haven't gotten around to it. The only original components on the bike are the brakes, front wheel, crankset, and front derailleur. I have an 8 speed nexus hub in the back (I had an entire new wheel built for the bike) and I didn't need to respace my frame to make it fit, but it is a tight fit... it's a very fun bike now.. I love how SILENT the internal gears are.