http://www.synthmania.com/t3.htmi think the timeline is as follows:
korg M1 (88) >
Korg T1/T2/T3 (90) >
Korg 01/W(91) >
Korg X3 (93) >
korg X5 (95) >
Korg Prophecy (95) > trinity
The Korg T-series is often called "the M1 on steroids" and it's quite true. The T3 features double the amount of ROM waveforms than the M1 has - 8MB as opposed to 4MB in the good old M1.
The T3 can also load M1 presets, and has the same 8-track sequencer - expanded and aided by a useful disk drive. The famous M1 high-quality effects are also present.
The presets have been reworked, to take advantage from the augmented wave ROM, and it shows - in all, even if this synth came out only a brief time after the M-series, it notably improves its sound and the performance capabilities.
Oddly, Korg never produced a T-series rack format, but the M1REX can be considered that - this rack contains the whole 8MB of memory that the T3 has ,and is a cross between M1 and T3, capable of morphing in any of these two fine instruments.