Just to clarify my questions, even though you have excellent recommendations now:
This may or may not be relevant, perhaps it might help others coming here in the future, but here goes…
(I did not use Dorico’s condensing, just copy/paste, paste into voice for these examples.)
Example A
This shows the same rhythms for each. I suspect this is what your teacher is referring to.
Shows [1] on a single stave. You can see the 3rds, unisons, spread (or not) of the note range of them. You can see the 2nds, so it is very easy to see the harmony.
Example B
Almost the same as [Example A], you see rests and different note values here and there, but mostly the same. You can definitely see how having them separated on their own stave helps seeing horizontally, where you might add slurs, add breath marks (if needed somewhere in a long phrase) etc.
Then collapsed into one stave, similar to your original example. You can see much of what you see in [2], 2nds, 3rds, the range of the notes (meaning the last measure has a little more spread) and unison notes. As you see bar 2 has a tie, bar 3 there are no 8th (quaver) rests, just the notes missing.
An alternative to [4] above where Dorico is showing each voice, so you see 3 horns with their own rests, beaming etc. Definitely getting a little messy.
Example C
Complicated, nothing much similar comparing each horn to any other
and depicted with separate voices
If your class is arranging, harmony, then if the rhythms are similar, working from a master score as you call it, is probably best, as your teacher has recommended. Easy to see your harmonies. Then explode to individual staves for further developments if needed, etc.
If you created them using Chords, Filter > Notes in chords > top note (second note or whatever) to select, copy/cut then paste to its individual stave.
For myself as @JustinM suggested, I would simply use 3 staves but ease yourself into this (read on.)
For different rhythms, you can see it might not easily work. It depends if you know what the horns are to be doing (thinking of when orchestrating) either working as a section same rhythms or near) or as individual independent instruments (for whatever reasons.)
Having separate staves for them will help if you are doing conducting classes or working with orchestra scores or groups of players like strings because visually you would want to practice with expanding your vision to encompass a bunch of staves vertically and horizontally at the same time, spotting the harmonies, 3rds, 2nds etc. as you “collapse” them in your mind. If you can do this as you are inputting your harmonies with them on separate staves, then all you need to do is condense for the full score. In creating a real orchestration, normally you would have horns on separate staves so their part is there for its player, condense as needed for the conductor score.
I think I speak for everyone here, have fun with your classes, feel free to bring up any questions you have from your courses, either related to Dorico, or any other music matter.