Multi level logic in transistor level. Possible?
In theory: yes

Is it practical ? No

All circuits are analog

Analog circuits have an infinite number of levels.

Digital 2-level electronics is an abstraction of the behavior of analog circuits. This is done because:


transistors are on or off, this saves power (current only flows during transitions)
this is simple, simple logic descriptions are easy to make, simulate, design with
It is robust regarding noise on the signals and the supply


Using multi level logic would get rid of most of these advantages and it would not bring a lot of advantages.

Multi-level logic would result in more complex and larger circuits, the equivalent 2-level circuit will almost always be smaller and therefore more cost effective. The exception is MLC flash memory but that is not combinatory logic.

Maintaining the "mid-level" voltages requires some current to flow, always, even when the circuit is static. A complex function would result in a very hot chip.