Here is a complicated solution: Instead of having two single players for the third trombone and tuba, you could have one section player using divisi. Use two lines for the names of the “instrument” held by this section player. Change the full name to Trombone over Tuba and the short name to Tbn. over Tba. Use divisi to create two section divisions. Change the label of the first division to III and the second division to a space. In the full score layout, allow section players to condense divisi. For the parts, create two layouts using this section player and rename them Trombone III and Tuba. Disable condensing in both part layouts and use manual staff visibility to hide the tuba staff in the third trombone layout and vice versa. Now the staff labels can look like this:
