I'm not saying that the balance is perfect but I think Mechanized Regimental Company offers the best options for dealing with this.
I am going to assume that before CP2 you get the tools you need to kill the M3A1. You can get 251/17 up very quickly with resource conversion.
I am also going to assume that it gets vet 1 because it is cheap and does considerable damage. 251's Vet 1 power is a Smoke Screen power that can protect it from Guards buttoning fire. 251 has greater range than Guards. It has far greater range than it has sight. With the right circumstances and with units screening and scouting for the 251/17 it can act as a good fire support weapon and still battle even when Guards are present.
The Puma also gets smoke immediately for OKW so you can escape buttoning fire from them as well. Puma wrecks the M3A1 so Guards cannot use it to chase. It's a fairly potent defense and the Puma scales up against the T-34 pretty well. You don't want to fight it 1v1 but it can easily turn a fight with support.
If you survive until CP 2-3 you now have better options for dealing with Guards as well. You've sunk fuel into your anti-tank and suppression weapons so you can sink manpower into Fallscrimjagers. Fallscimjagers can kill snipers from behind pretty well and I bet they do pretty good against Guards too. I find that Fallscrimjagers are a little inconsistent and much prefer Obersoldaten but I think the early fuel investment will push you away from them. The MG-34 would also help deal with Guards as their immobility makes them a weak flanker.
You might be able to make other doctrinal infantry work as well. I've heard good praise for G43 Panzerfusiliers but if you're converting out of munitions and probably using smoke rounds so I don't think Breakthrough Tactics will offer much aside from the Sturm Offizer.
I've come to rescind this opinion. I was wrong.
I was just wrong:
1) Relying on any weapon team to defeat M3A1s is foolhardy. Snipers offer the perfect counter to every single early game play. Building a Kubelwagon is suicide against Soviets due to how effective the M3A1, Maxims and Conscripts are against it. Early game snipers are very hard to oppose.
2) You can totally lose map control before the 251/17 even arrives. M3A1 + Conscripts + Sniper is enough to force anything off if played well. Flamethrower comes out early and trivializes the DPS of Sturmpioneers against the M3A1. Snipers throw this fight even more out of balance.
3) Only an idiot doesn't go Tier 2 after Tier 1 Soviets. This build is stupid good against OKW. You get the suppression the Maxim covering the Snipers that invalidate elite troops with Conscripts everywhere. Early half-tracks are forced out or killed by ZiS guns. 251/17 can't escape a ZiS gun if it gets caught. The normally high reload time of the ZiS-3 is meaningless thanks to the "unsetup time" of the 251/17.
I had to go against Soviet T1 + T2 into Call-ins twice and I switched between two strategies and absolutely failed to anything with both. Soviet call-ins are stronger than ever with the superior IS-2 now in the battle. You can battle the enemy with your superior manpower force for ~20 minutes and then flatten what pathetic hope they have with mass medium tanks or heavy armor. God forbid he goes with the ISU-152 and you don't have Panther tech. Jagdpanzer doesn't stand a chance and has no flanking power.
Snipers have never really had any good hard counters but for the new side this brings it to a new level.
What fast unit does OKW have that can harry this unit?
Puma has no anti-infantry power except at point blank range. Aggressive actions are easily countered with AT grenades.
251/17 Flak AA Half-track has to setup to fire. You can just retreat early game and then later on it is no good at aggressive flanking actions.
Kubelwagon has joke durability and poor accuracy while moving.
Panzer II "Luchs" was moved to the Tier 4 building so it requires a bare minimum of 130 fuel to bring out! It's still fragile as hell and dies to a mine + AT gun shot.
It's like Fallscrimjager flank is the only way. You can't control what exit of the building your Fallscrimjagers get out of so even when you
do have the right building access there's good way to do it. This is assuming you even have a building for the flank!