Yea, theoretically, it is possible to get the Armor from Romania and/or Hungary and/or W. Poland to Moscow in 3 turns. Actually in 2 turns.
Realistic? I hardly think so. Your best real chance is at a minimum of round 4 and more likely round 6 if you declare war on Germany 1.
Which is a thought, has anyone tried declaring war on Russia round 2/3 and started an invasion early? I wonder On round 1, it is theoretically possible to kill 7 Russian infantry, sink a Battleship and a submerge a submarine (unless he wants to stand and fight) and take 3 Russian territories.
Of course, what would you have to give up to do it?
I think you wouldnt have to give up too much if you got lucky :-). And getting Moscow by G3 is definitely unrealistic…