Why do tail dragging seaplanes not exist?
It's been tried before (there isn't all that much in aviation that hadn't already been tried by WWII).  The tail float means the aircraft CG is aft of the hydrodynamic center of the main floats, and like a tailwheel airplane, the machine will be dynamically unstable, steering wise, when on the step, same as a wheeled taildragger and you will have to prevent it from switching ends with your feet the entire time it's on the water moving.
It could be a real handful to take off and land, especially when there are waves and gusty winds (and seaplanes do have to cope with crosswinds from time to time).  You have to make the rear float steerable, and you have to give it enough lateral bite in the  water to have good leverage in steering the thing when below the speed that the air rudder is working.  I don't think there is any drag advantage to 3 smaller floats vs 2 larger ones.
Traildragger advantages, like prop clearance, aren't that much of a benefit here, and since the conventional float configuration is so much easier to control, why bother with a taildragger float setup?
Ideas come along for new configurations all the time, but if you dig enough you find that somewhere in the distant past it's already been tried, and while it may have been functional, the &quot;old&quot; conventional way that everybody uses has been around so long because it's simply what works best.