Where do you put the third wheel - in front or in back? A one hundred and forty year-old question!
So here we have an example of each in tandem form (the rear wheel of the second machine - blue arrow - is rather obscured). You'll see the problem right off: one of the riders is outside the wheelbase, risking going over forwards* or backwards on slopes, potholes or under braking. To counter this, small caster wheels (red arrows) were added. These were positioned above the roadway during normal operation.
You'll note both gentlemen having to look
around the ladies!
*I bet that ruined the date!