Just posted a 3x comparison online, the is a brand new test and includes the following lenses:
Canon 35MM f/2.8 Macrophoto Lens
Canon-MP-E 65mm f/2.8 1-5x Macro Photo Lens
Minolta DiMAGE Scan Elite 5400 Scanner Lens
Mitutoyo M Plan Apo 5X 0.14 objective @ 3x
Nikon 3x Measuring Microscopes Objective
Olympus Zuiko Auto-Macro 38mm f2.8 <not incl. in the above image>
Qioptiq Mag.x LD Plan APO 5x / 0.20 Objective @ 3x
Schneider Kreuznach Componon S 50mm f/3.5 Fixed Aperture lens
Tominon 35mm f/4.5 Lens
The biggest surprise was the performance of the Minolta Elite 5400 lens. The Qioptiq and Mitutoyo come out on top but the Minolta is surprisingly close to the M Plan. You can see all the details here: https://www.closeuphotography.com/3x-lens-test