i doubt it. it worked vs ai because ai spams attacks with battleplan so much. a guy in mp who knows about mass mob --> roaching will probably notice the bizarre division and would certainly decline to attack into it the way the ai does. unless you deplete first, any offensive action would immediately get counter-pinned and punished in trades because once you move stuff and lose entrenchment, i think mass mob would trade favorable in ic. it was melting the ai due to entrenchment. it is a very swingy division.
Hey, replying just in case you were interested in the results. Just tried 3/3 offensively against a human player going mass mob and your comment was completely correct (japan vs china), I mixed a good chunk of nomal defensive infantry too to not just intsta collapse against their people bashing into me. To give a few details on the 3/3 performance:
1. At first I could take the initial tile easily, say a 70% chance to win a tile quickly in the beginning of war (if he didn't notice the push and didn't reinforce/counterclick) and later I felt like my offensive chances per tile were 30% in good supply and 0% chance in bad supply/river for me to inflict enough soft attack to cause a reinforce meme (the initial success could also be because it took him a while to get the massive reinforcement from doctrine and guerilla warfare too).
2. However, the counter clicks from the masses were brutal, even if I could hold the tile I pushed, it came at a cost of some hp which translated to a cost of veterancy and overall slowing down offensives.
3. I eventually could not ever push again, not because of lack of equipment but the lack of veterancy and the enemy player counter clicking/pinning (exactly as you mentioned btw) meant eventually I started losing tiles and it became a tug of war that I feel I would have lost. The enemy got massive losses yes but it meant little against the millions.
4. Supply and rivers, the second the 3/3s met any river even in great supply it was like their damage became null and they lost a lot of hp and veterancy. Same for low supply, the second the supply was even a bit dark orange they also could not do anything. I am not as good as you so my guess could be wrong, but I imagine it is because of the lack of breakthrough, and those negative modifiers affecting my breakthrough even further (and the soft attack too not letting me win quickly which is the whole point of the 3/3s, if battle ends quickly, hp and org should not matter in theory) meant the 3/3s really quickly were kicked out of battle 1 by 1.
5. Denfensively, the 3/3s were fantastic. You are really smart, as long as I had entrenchment and the 3/3s were selected for battle the enemy infantry could do nothing at all, enemy deorged extremely quickly and my losses were practicaly 0.
Tl;dr. 3/3s offensively good at first, if enemy figures them out they stop suiciding into them and counterpinning effectively (like you predicted), eventually 3/3s can't take a tile tho because of being pinned and even losing after they have no entrenchment/planning or the battle lasts too long because of reinforce rate (low breakthrough and org meant the 3/3s with a long enough battle eventually stopped winning even if it was very favourable at first) . Defensively they are really good even against a META massive infantry attack, I was really surprised. You are a genius for already knowing exactly what would happen btw but your 3/3 division did stop the massive infantry 30+width effectively in favourable defensive conditions, however those 30+width still seem like the best offensive thing if you have no tanks/special forces, alas, the 3/3 could not make artillery as a batallion the new secret super powerful meta division despite my hopes.
A quirk that I think you may find interesting: when I had to defend, we noticed that the game prioritized my normal infantry divisions to send them into battle instead of the 3/3 despite my 3/3s having signal companies (which would mean the initiative for them to be prioritized but no?), it happened many times and we still do not know what caused it. This meant most defensive battles half or more of my 3/3 could not even do their massive soft attack damage due to being in reserve. I am not sure if just making the entire army 3/3s would have worked better. I had 30 3/3s at the start of war, the rest were normal infantry, just to give total numbers. I feared he would just battleplan me to death, that's why I didn't make that many 3/3s. Again, I don't know if a pure 3/3 army would have changed anything. I had air too.