I have a 10.5-inch Reed 2A mounted on my Mk3, and it is the best of my many tt/tonearm combinations. But really this has been said many times here, the tonearm should "match" the cartridge, first and foremost. True, the tt will impart its own set of colorations but in my experience, those are rather independent of the tonearm chosen. You cannot use a 9-inch Triplanar on the Mk3 without re-plinthing the motor separate from its square escutcheon-like chassis, simply because the offset of the Triplanar base from its bearings gets in the way physically. Same would hold true for a 9-inch Reed or possibly the Talea. (Talea is a shade longer than 9-inch I think, so might work.) 10.5-inch and 12-inch tonearms can easily be mounted so just choose one that is a good match for your cartridge. Then fugeddaboudit.
Ralph, Can 9-inch Triplanars be "upgraded" to the 12-inch version?