Linkage adjustment can cause it to not shift to 3rd but linkage won't likely prevent it from downshifting at a stop.

If it hangs in 3rd at a stop but shifting to low will get 1st gear and the cycle repeats, the problem is likely a stuck shift valve or governor. The only way to know for sure is to connect a pressure gauge to the governor test port, anything over 2 psi at a stop in Drive is a governor problem. If the governor pressure is OK and it won't downshift at a stop the 2-3 shift valve is stuck.