The M3 armor was nerfed while one of it's main targets (the pioneers) damage was increased. That is what's making the M3 underperform. The video examples illustrate this clearly.
Now without pointing fingers on anyone, saying those videos are stupid is frankly a stupid thing to say. Their purpose is to isolate the unit in a favorable or unfavorable situation and test it's ability.
To think I will use it exactly the same way in the video is wrong.
That is exactly the problem there. The whole system was changed, but you seem to have not adapted. Pios might've been a primary target for a M3 in the old one, but I wouldn't say they are anymore. Also Pioneers are a combat unit now, not 'just' a builder. You can still fight Pioneers, but only on range.
Pio DPS with 100% accuracy:
08 15.435
15: 7.900
30: 2.726
As you can see there is a huge difference in DPS even at range. Full auto units shoot smaller bursts and slower at range, not only with less accuracy. If they are moving the bursts tend to get even smaller.
Great targets for M3 currently are Grens at all ranges. Close range will get you fausted usually, but depending on the situation that is often not bad. Pios on long range work good as well. PGrens are okay - they will deal a lot of dmg, but also bleed quickly. Snipers are easier to kill than ever.
Besides Pios in close range, LMG42 Grens are a dangerous prey due to their higher penetration. I think the biggest problem with the M3 currently is that people drive close. Keep it more on range and only swoop in to finish off, to make him faust you or to push him out of cover if you must. I think it might need some minor buffs and I would like it rear armor to be the same as the frontal one (since it's non turreted and the autorotation is sometimes annoying as hell), but besides that I do not see much problems with the M3 itself, but more with the 222 which is too strong atm in my opinion.