From what I understand, the 3 jackson idea is more for larger teamgames. In a 2vs2 or a 1vs1, 3 jackson is just asking to throw the game. I would recommend 1-2 Jacksons + 2 Shermans or 2 M8 scotts + 1 Sherman + 1 Jackson. I find that lategame it may be worth your money to buy a new m20 to lay mines, as a crippled panther means that your armor will have a much easier time taking advantage of their range. Noone expects an m20 lategame, so try to mine in areas that he will have no sight on you, and keep the m20 decrewed in base when its not being used (though you can use it to harass lone capping squads if you wish).
2 kiting shermans will munch any volks blob, regardless of unit numbers (within reason).
Usually if I am facing heavy tanks in 1v1 I go with a Single Jackson because having more then one hurts you hard in AI and the upkeep is terrible.
I also keep the Jackson uncrewed at base until its time to bring it up. Shermans are great tanks but not cost effective against Ostheer Tigers that are supported by Paks. Its very very easy to lose them and they cant fire from a safe distance. Further RNG can screw you over and you could lose some of your Shermans without a Tiger kill even with as many as Three trying to kill the thing. Gotta remember that although a better tank the Pen on the Sherman is the same as a T34/76.
I think the Tiger and the Heavies arent as much as a problem as the Panther for OKW though (in 1v1). Because it pretty much invalidates your Jackson and any other armor due to its massive Armor and HPs. It also delays your Vet due to high bounces.
To deal with Blobs I agree M8 is wonderful. Two is even better. The M8 more then anything rewards USF micro the most. The Jackson does against Mediums but not against Panther and up. Its more of a deterrent then anything else. And once again back to the Panther since its faster then the Jackson and it gets Blitz at Vet 1 it is especially aggravating further reducing your ability to get a kill with your Jackson.
I would say that I have killed more Tigers with the Jackson then I have managed to kill OKW Panthers with it.
I would like changes to it as well. Even if a cost increase was involved. Simply because its the APEX unit of USF. I like the idea of less damage but better Pen as this would result in getting vet faster to make it more rewarding to use.