"MT3 Profile" refers to the physical height and shape of the keycaps. Some features of the MT3 profile that make them stand out from the more common OEM and Cherry profiles:
High-Profile Keycaps: Keycap height is significantly taller compared to keycaps in OEM profile (and Cherry profile, of course).
Concave Surface: Indent starts from the four outer edges and curves softly towards the middle of the keycap.
Rounded Corners: Gentle and finger-friendly, lends the keycap an almost cute appearance?
Visual Comparison of Cherry, OEM, MT3, and SA Profile Keycaps:
EschewMT3 profile is the best profile. Cherry and OEM profiles are boring and uninteresting. And SA is nice, but it only appears to come in SA variants.
Normal keycaps you'd buy are a single piece of plastic with the numbers printed on. These can fade or chip off, so another technique that is considered higher quality is to instead use one shot to make the keycap with voids for where the character would be. Then a second shot of plastic is added to fill that void and create a keycap much less prone to faded/chipped lettering.