Pilot reported a NMAC with an aircraft conducting a non-standard pattern entry and cited an overly congested CTAF shared by nearby airports as a contributing factor.

Narrative

In coordination with my Flight Instructor I was on a solo cross country to MCX. While departing from the MCX airport on upwind; another aircraft (soon to be a conflict aircraft) entered the pattern at a nonstandard entry or location. Their location was unclear. At this time; there was myself; a second aircraft; and the conflict aircraft. The conflict aircraft was unclear and they were either on a widened downwind or extended upwind or extended downwind. Their location was unclear. To further complicate the issue; the CTAF being used is on the same frequency as several other airports; and the radio congestion was heavy between the different airports and aircraft. I was safely maneuvering my aircraft in the appropriate standard pattern operation and diligently looking for the conflict aircraft that was in the unclear location. The conflict aircraft appeared to have me Insight; however; they did not adjust their flight path to stay clear of myself. Suddenly; that conflict aircraft that was in a nonstandard pattern and unclear area; came very close to my aircraft. I maneuvered my aircraft to avoid.Inadvertently we came within a couple of hundred feet of each other. After this I returned to the home airport. I believe that this situation would not have happened if the other aircraft; who was entering in the nonstandard location should have appropriately exercise the requirement to see and avoid. If they could not maintain a safe distance; then they should not have entered the pattern. If CTAF frequencies were specific to an individual airport and not commonly used by adjacent airports; then this would've greatly enhanced situational awareness.