The most significant difference between these three options appears to be the power of the built-in sound system.
Mark-Pro ONEiF: Built-in, 15 watt rms per channel stereo amplifier:
(2) 4x6" full-range drivers
Mark-Pro TWOi: Built-in, 30 watt rms per channel stereo amplifier:
(2x6.5" woofers; 2x2" tweeters)
Mark-Pro THREEi: 80-Watt Amplification: 2 x 40 Watts RMS Per Channel
4 Speakers: 2 x 6.5 inch (woofers), 2 x 2.5 inch (tweeters)
There are other differences; the number of selectable sounds; the number of drum patterns; the number of sequencer recordings; but if your intention is to play it as a piano, the amplification and speakers would seem to be the important thing.
The keyboard quality, and the quality of the built-in sounds, I would guess is the same on all three.