The M3s were not like a low mileage used car. Any armoured vehicle of the time wears out fast unless you indulge in expensive maintenance and replacement parts. Much to the annoyance of the Americans the British found it easier to convert newly delivered M4s into Fireflies and specialist vehicles even though they had older ones already delivered.
By the time that used M3s might be used for improvised APCs the production lines had gone over to M4 production. It was a selling point for M3 production that the same chassis line could easily make M4s later.
You would have to use what you already have as AFVs going out of service. In an M3 the turret has to go as a huge space waster. The only place for a gun is in the sponson mount and that has to be one with a short breech or it too will be a space waster. Hence my earlier offering of 3" Cs gun or some pack howitzer. Poor weapons but ones that can put down some HE and smoke in support of the dismounted infantry.
IOTL we find that what was used was simply existing ones with the turret or artillery piece removed. This was easy, fast and used existing supplies. To expand it further you need to identify other obsolete AFVs to bulk out the OTL numbers. The M3 is an obvious one of these for M4 mounted divisions and Cavalier/Centaurs of Cromwell equipped ones. Even Crusaders (shorn of their turrets for heavy armoured cars). Recce units were taking turrets off Honeys as the armament was not worth the high profile so I can't see the point of keeping an M3 turret.
Unless we can find a POD well before M3 production ends then you will not get IFV designed M3s. Also we would need the POD to include extra production lines to maintain the OTL M4 production levels and that type of heavy engineering was fully tapped in the USA and Canada. I would suggest that an early IFV TL would only be economically possible with a wheeled chassis for which the Staghound gives us a model. Definitely not the complex 8 wheeled German jobbies.
By the time that used M3s might be used for improvised APCs the production lines had gone over to M4 production. It was a selling point for M3 production that the same chassis line could easily make M4s later.
You would have to use what you already have as AFVs going out of service. In an M3 the turret has to go as a huge space waster. The only place for a gun is in the sponson mount and that has to be one with a short breech or it too will be a space waster. Hence my earlier offering of 3" Cs gun or some pack howitzer. Poor weapons but ones that can put down some HE and smoke in support of the dismounted infantry.
IOTL we find that what was used was simply existing ones with the turret or artillery piece removed. This was easy, fast and used existing supplies. To expand it further you need to identify other obsolete AFVs to bulk out the OTL numbers. The M3 is an obvious one of these for M4 mounted divisions and Cavalier/Centaurs of Cromwell equipped ones. Even Crusaders (shorn of their turrets for heavy armoured cars). Recce units were taking turrets off Honeys as the armament was not worth the high profile so I can't see the point of keeping an M3 turret.
Unless we can find a POD well before M3 production ends then you will not get IFV designed M3s. Also we would need the POD to include extra production lines to maintain the OTL M4 production levels and that type of heavy engineering was fully tapped in the USA and Canada. I would suggest that an early IFV TL would only be economically possible with a wheeled chassis for which the Staghound gives us a model. Definitely not the complex 8 wheeled German jobbies.