Train is generally very clean - infact both Mussoorie and Nanda Devi both are one of the very few trains in NR that are well maintained.
I have travelled mostly by 1A and 2A and it was clean as I mentioned so I assume should be same with 3A as well. However remember at times this train reaches both ways destination with a delay, if delay is not a concern (not everytime though) then it's a very good train to reach DDN/DLI (either side).