If you powered this thing for longer than ten minutes, you'd have to start moving forward to push the air over the motor to keep it cool enough

Easy enough to solve: use each idler to drive a fan. Also, large radiator. Where? Everywhere. The whole bike. Heat-resistant clothing mandatory and no shoelaces allowed!

how do you sync idle speeds and direct engine rotation for three different engines using a direct timing chain and no clutch system to deal with any differences in engine rotation?

You assume there's no clutch system, but perhaps there's no throttle system! When you start the bike, all three engines scream, locked at full throttle. The handle that would usually control throttle instead is directly tied to three clutches and your speed is determined by how engaged your clutches are.

People will always know when you're approaching: first they'll hear you, then they'll smell you, and then they'd see you.

how do you sync idle speeds and direct engine rotation for three different engines using a direct timing chain and no clutch system to deal with any differences in engine rotation?

*Spoken as Marisa Tomei* Positraction. I refuse to say more because whatever implementation your brain pictures is far better than any mental image I can paint with words.

Show, don't tell. Release the butt cut!