First of all, to engage third gear the engine or rear wheel needs to be turned a little bit to and fro in order to be sure that the dogs does not engage. It is quite tight play in the dogs so they don't slip in automaticly.
If third gear still does not engage, you can peer into the gearbox through the shifter tower and watch the slider gear dogs whether they engage the third gear or not. It takes just a very small distance travel for the 90 degree lever (20 B 5) down there, between a failure to engage or engage.
If third gear still doesn't click into position with the 90 deg. lever touching the lid tower at its end of travel, the selector fork (20 C 8) could be bent. Might possible be felt with the shift stick as a early engagement of the first gear.
Both ends of the 90 deg. lever can be inspected when the gear is in 3rd position. The fingers on the lever should fit the selector pin quite snug. And the long shifter lever (28 B 79) -you just removed- should fit the other fingers quite snug, with just a small play.
A long shot could be if the 90 deg. lever would be bent downwards OR if the nut on the top of the lid is loose. If that is the case the 90 deg. lever could touch the top of the gearbox housing and prevent enough travel for the third gear to engage.
Another, but common problem is a sloppy or sheared location pin (28 B 81) for the long lever/shift stick shaft (28 B 80). There is tremendous forces at play on that pin. A worn pin is felt by a large slack in the shift stick.
If all checks out, third gear engages and it still feels a bit funny, check that the chain sprocket hasn't come loose! A very useful tip that could save a lot of grief and money is to use a safety screw and washer on the sprocket to secure that the sprocket nut stays tight!
