The station problem is easy. Make it like X-Flight/Batwing, except 3 tracks. Just have different floors for the proper train.
The problem would come between the stand-up and the floorless/sitdown. Stand-ups have a different center of gravity than the floorless and sitdown. To keep it rideable for all 3, it would be very, very boring. A few small hills up, a few small hills down, maybe a vertical loop if the trains all could get enough speed on the hills without putting too much stress on riders.
Another problem would be inertia. Stand-ups are largest, and have the most mass (I think), followed by sit-downs, and then floorless. This would change the speeds throught the ride for each train, meaning a gentler layout so all 3 can get around the track.
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Everybodys got something to hide except for me and my monkey!