I recently picked up some MX3 on sale out of curiosity and I figured I'd post my results. It's supposed to be several degrees better than AS5 and though I can't conclusively confirm this, I do think it's probably better. Nothing too exciting here, and no real new info over the numerous comparative reviews, but I was pleasantly surprised given that I went in expecting no difference.
I tested on my system, which features an i5 750 cooled by a CoolerMaster Hyper 212+, all at stock. This is a 4x6mm heatpipe direct-touch cooler. The paste application method was 4 thin lines along each pipe. The Arctic Silver had been mounted and nicely settled for a couple months prior to test. The MX3 has been on a couple days - Arctic Cooling claims no settling time to I'll hold them to it.
Note that the MX3 delivers comprable temperatures in spite of a higher system temp and with a fan speed over 100RPM slower. Nothing really scientific here, and not stupendously exciting results, but I'll take 'em.
My only beef is that the syringe in which the MX3 ships makes it hard to apply fine beads of paste in direct touch heat sink situations. It's much better suited to big blobs than fine lines.
I was re-testing AS5 vs MX-2 a few days back and i can confirm that MX-2 is better that AS5. I don't know what they did to AS5 in recent months but i noticed it's very dense - way too dense IMO which makes the application a PITA even using the "blob" method.
I don't know if MX-3 is better than MX-2 as i've never tried MX-3 myself.