3 Infantry + AA Gun (upper limit you wanted)
3 Infantry, Artillery, Battleship attacking.
No AA Gun shots, no fighters present.
Attacker has 86% chance to win with most likely outcome of losing 2 infantry in the attack.
Russia has lost 3 infantry and an AA Gun which is now poised to attack Russia if they send fighters. If they do not send fighters, they now have to send some hard units (as in not flesh and bone) to recapture the land that will be open to attack.
I don’t think this is a winning proposition for the Russians. As Germany only needs to make sure that they force Russia into risking fighters to AA Guns or sending artillery/armor into Ukriane to be destroyed by the Germans.
Honestly, if I was left with Inf/Art in Ukraine, I might send in two more infantry for reinforcements. That would leave Russia facing 3 Infantry, Artillery and AA Gun. (Or I might just send an armor instead of the artillery in the original attack, even though it only increases the win percentage by 2%.) That would almost require Russia to send at least 4 ground units and/or planes. That’s generally most of their build per round.