First of all, the tank destroyer units had priority over armor in getting "limited" Ap rounds. So assume the m36s, which were produced for the very purpose of countering Panthers and Tigers, will have these available.
Wich other vehicles used the M3 gun?
Exactly one! The M26. Wich was fielded in a two digit number in late 44 until the war in Europe ended.
So there is no prioritising. The 90 mm ammo was only really needed for the M36. Yet the M304 was limited issue. Wich means only a few rounds per platoon will be available if any.
Limited issue doesn't mean the few that are available are evenly distributed. It can't be assumed that any M36 equiped unit wich faced Panthers or Tigers had them available when they needed it.
I did a little research and found no evidence that any HVAP shells for the 90 mm guns were actually issued to M36 equiped units during the war. It seems this ammo was specifically designed and produced for the M26 and only issued to these during WW2.
Second... if you answer is going to boil down to "historical"...
You completly miss the point.
My answer to the statement about the 90 mm M3 gun outperforming the 7,5 cm KwK 42 is using historical facts. In this case the answer is that this statement is plain and simple wrong.
Game balance wise I allready posted that I do think the M36 could need some improvements.
The limited availablility could be refected like the limited issue ammo of the AT57.
Actually I think making the M36 THE bane of heavy armor in this game would be a good idea since the US don't have heavy armor on their own. So getting the best hardcounter against it makes a lot of sense.
That doesn't mean I think a single M36 should ba able to slug it out with a Panther in a direct head on duel. The M36 should stay a tank destroyer. Even if it becomes the best one.