My two cents:
Per
Rule 7.1, "Each unit which elects to "FIRE" during its activation segment may perform
one type of fire." [emphasis added]
Per
Rule 12.11, "good-order units . . . may enter an adjacent hex occupied by enemy units. This is called "assault movement." . . . This initiates an assault (or reinforces an existing one)."
Per
Rule12.2, "Assault combat must be resolved immediately when active units enter a hex containing enemy units and no friendly units. Resolve the assault after all active units which are to enter the assault hex this action segment have finished entering the hex.
All units of both sides in the hex must participate."
Referring back to 12.11, "Units which begin their activation in an assault hex or enter an existing assault hex are not required to assault, and can opt to recover morale, leave the hex, or do nothing (12.3)."
Put those together and it seems to me that if you initiate an assault,
all units that move into the assault hex in that activation
must participate in Assault combat using the Assault Table. If they begin their activation in an assault hex, they may
either fire with AT fire at armor in the hex or assault. They may not do both. Infantry AT weapons in an assault hex are a special case, spelled out in SSRs.
If assaulted, the defenders all participate in Assault Combat using the Assault Table. Since defending does not count as their activation, they may (on their activation) either assault or use AT fire against the enemy in the assault hex (or recover, exit the hex, dig-in, or just sit there).
In the example of the StuGs assaulting the T-34s, when the StuGs move into the hex, they all
must assault.
All of the T-34s will defend, and both sides will use the Assault Table. Assuming they all survive with no disruptions, the T-34s on their activation may
1) All three assault the StuGs, or
2) All fire at them with AT fire, or
3) One fires using AT fire and the other two assault, or
4) Two fire using AT fire and one assaults or
5) Use a Move activation to dig in, exit the hex, etc.
In (3) or (4), the Russian player may activate all of them together in one activation, and choose which to carry out first, the AT fire or the assault.
Assuming all three StuGs survive with no disruptions, on the following turn the German player will have the same choices as the Russian player did.
