The Mach X 3 is a plated long-distance trainer: propulsive enough for marathon race day, durable and comfortable enough to train in. A good answer if you want super-shoe feel without committing to a fragile racer.
Heel lockdown is secure with minimal slippage, and the heel collar is more traditionally padded and flared away from the Achilles than on the Mach X 2, which removed the rubbing complaints of that version. The midfoot feels slightly snug on step-in and then settles once you are running. Sources disagree on forefoot width, so treat it as a snug performance fit and try it on if you are wide.
Best for: Long runs, tempo work, marathon race day
Sizing: True to size
Fit: Snug performance fit; check availability before counting on a wide