the big question is how much were you expecting the night train to be? I assume you were using www. nightjet.com .Did you try to get a price for one person and think it was high, or did you actually ask for 3 -because doing that it quotes for 3 not each. Nightjet prices adjust airline style based on demand so you must ask for the actual date you want even if just checking prices, if you've left the date set as "today" it will be expensive, assuming that it isn't sold out! And if you just want ot sit up all night and try to sleep you could do that on Flixbus for a lot less money (and a faster journey).
Bayern Tickets are never valid on long distance trains, so not even usable on these - no chance of discounts. If you use one of those to take a late train to Salzburg and catch the night train there it would be cheaper - but you lose several hours chance of sleep. And if you only used that day ticket for the ride from Munich to Salzburg you'd have to deductthe cost form any saving, so even smaller!
Vorteilscard doesn't seem to get any discount on this either, and it would cost €21 each.
Other tips:
remember that Bayern Ticket is only valid after 09:00 on Monday-Friday (except holidays). And you will buy ONE for 3 people, not three individual tickets. Not valid on IC/ ICE/ EC/ ECE/ RJ/ RJX trains - or NJ Nightjets!
if your day is wholly within Greater Munich there will be a cheaper MVV ticket
if your day doesn't need buses or trams or U-Bahn the Guten Tag ticket will probably cover it for slightly less.
Bavaria is bigger than some people expect. As an example it has a land area slightly bigger than the Republic of Ireland and it's population is 10 million more than Iowa! So think of it as a country (which is what a lot of it's people do....). Munich will obviously be more expensive than smaller towns and rural areas - so given the quality of public transport try somewhere on the fringes.
