The strat is annoying but far from unbeatable. Personally my favorite counter to it is to go Gren, Gren, Sniper (if you like playing elite troops you can vet it, but it isn't required). If they're inside an M3 they have no control over what their sniper targets, and they cannot hold fire, which makes it *VERY* easy to countersnipe them. Just run any squad into range of their sniper in the M3, it shoots, you shoot with your sniper. Ez pz. The only problem is that currently snipers in M3s are bugged and you can *NEVER* kill the second member while it is in the M3 haha.
Anyways, even if you can't pick off both sniper models (for now) you can still snipe guards all day to bleed the heck out of them. As long as you're careful and don't derp it should be almost impossible to get countersniped.
Alternatively you could play the A Gren commander, spam some grens early game and then wait till you unlock the 250 with Pgrens inside. This baby is so good at chasing down/killing M3s and Soviet snipers. Also it doesn't really care about being buttoned that much because it has the same health as a 251 plus the Pgrens will just shoot the guards while it is buttoned. Only thing to really worry about is hitting a mine but it is pretty easy to sweep the common areas before you rush in.
Another thing that has worked for me is the mortar half-track. The mortar HT has no setup/teardown time and is highly mobile so it is really easy for you to just follow his M3 around and shoot at it, given that sniper/guards can't really win until 8 or 9 CP (and you don't need to fear a quick T70) it is only a matter of time till your mortar HT instagibs the sniper squad. It also happens to be extremely effective against Guards since every shot that hits them is going to bleed heavily.
Also also, get creative with using LOS blockers. Hide grens behind bushes/houses/etc. I've lost track of the number of M3s that have driven around a corner and right into my faust. You can even bait this pretty easily with an MG. Since he won't have very many flanking units, 95% of the time when a sniper/guard player finds your MG - he's going to send his sniper to kill it, use this knowledge to your advantage and set up an ambush.
Finally, 222s - if used properly are a fantastic area denial tool. They can keep the M3 in check, they can prevent it from chasing down retreats, and they force a guard squad to ALWAYS be around the M3. Additionally if you ever land a nice rifle nade/MG pin/etc and force guard away it can be a good opportunity to chase the M3 down. I see so many people completely derp with their 222 use and then cry imba. Sure I think the 222 could use some love, don't get me wrong. But I frequently manage to keep 222s alive until Su85s hit the field. They're quite effective on maps like Langres/Minsk
Mortars are completely RNG dependent and require a "lucky" hit, does seem like the best counter. However the scout car offers protection from mortars. Is it true the second model never dies!?!? WHAT?? So it is an exploit!!! Ugh ...
Flanking and hiding are good strats, however very good players stay away from those blinds spots and use cons/guards to scout. That's what I do, I am 4-0 with this cheapAss strat.
One thing you forgot to mention is the at gun. Though not the most reliable it can kill that pesky scout car and open up more opportunities to get a mortar hit. HOWEVER, by this time he is well on his way to katuysha or sU-76 and has been contesting your fuel while keeping his.. Plus hAs been draining your mp for first 4-7 minutes of the game.
222??? No sir, completely unreliable. It's a tin can. If your opponent messes up royally then I guess...
So basically it takes skill and a lot of luck to beat it. Once you can kill the scout car, especially early, there is a better chance.