So I opened the troubleshooting manual you attached and that is a useful article for sure. It explained each gear process individually and it said:
3rd Gear
In 3rd gear, brake band B-2 is applied and clutch K-1 is
engaged. Only the rear planetary gear set is involved in gear
reduction.
4th Gear
In 4th gear, clutch K-1 and clutch K-2 are applied. Both
planetary gear sets rotate as a locked unit.

If I understood this correctly this says that K1 is used first for third gear, which I have lost. It also says that K1 (and K2) are used in 4th, which I probably have not been able to go to since I lost the ability to connect in third gear. So if this makes sense, I am thinking that I need to focus on the K1 portion of the transmission and I changed the K1 spring set before this happened. I did note that the kit I got did not have a new piston or new piston rings (seals). I am wondering if that spring piston set in the K1 port on the valve body that I pulled and changed has something wrong with it now. That might explain the suddenness of third just going away while driving. Any thoughts?

__________________
Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red