Problem with 3rd Gear?

2004 Acura TL, ~158,000 miles

so I posted here a while back that my car was shaking while accelerating, and I got some different responses about what the problem could be. Today I put the car in manual mode to see if I could figure out any more about the issue

The problem (seems) to be when I am in third gear. If I am going up a hill or something in automatic the car will start to shake like it wants to stall or something until I take my foot off the gas, or if I accelerate hard it doesn't seem to happen either. Only if I am accelerating at a constant rate, it will start to shake, and I assume now that it happens when I am in third gear because it usually happens when I am going uphill in low gear, and I believe when I popped it into manual last time it only happened when I was in third gear as well.

Also shifts in and out of third gear have rougher transitions, and when I am in third gear and let my foot off of the gas the car begins to heavily slow down almost as if I am putting my foot on the brake or there is heavy resistance to the current speed. It doesn't happen in any other gear. It just drives differently if I am in third gear, and I don't seem to have any of these problems any other time.

Any help would be greatly appreciated.

Thanks,
Dan