If you can spend a day in the Frankfurt area right after arrival, that would allow you a sufficient time buffer to take advantage of the train (time and date) specific, advance purchase, Sparpreis fares as low as 20 euro pP. Elsewise, you will need to spend 105 euro per person for 2nd class in a direct high speed ICE to Munich. That trip will take 3½ hours. Or you could save 19 euro pP by taking slower express train like Intercity (IC) or Eurocity (EC) with multiple connections in just over 5 hours. There are other, less expensive but slower options.
With the right airline, you could have booked a Rail&Fly for 33 euro for an open ticket vallid on any train, including the ICE, to Munich whenever you arrive, but that's probably water over the dam.
You can still look for a Sparpreis ticket from Munich back to Frankfurt for as low as 19,90€ pP.
The Dachau trip is just a local trip within Munich, although it is outside the innenraum (inner zone). For that use a Munich XXL Group day ticket for 16,10€. That is an all-day transit pass for all three of you in the inner two zones of Munich. It's valid for both the S-Bahn to Dachau Bahnhof and for the bus from there to the Dachau Memorial.
For other days in Munich, you will probably stay within the inner zone, so Innenraum Gruppen Tageskarten (inner zone group day tickets) will be all you need. at 12,80€ per day. You could save a little if you put your three "explore Munich" days together and buy a three (consecutive) day inner zone group ticket for 29,60€.
The Salzburg trip and the "castle" trip can be done using a Bayern-Ticket (37€ for 3 people). This is an all day ticket (after 9 am workdays) valid for unlimited travel on regional trains (and most buses, such as the bus from Füssen Bhf to the castles).
As for using the Bayern-Ticket and Meridian regional trains to Salzburg versus more expensive trains with a railpass, the time difference (~1½ hrs vs ~2 hrs) is not, IMO, worth the added cost. Also, the Bayern-Ticket is valid for any transport in the MVV (Munich Metro), S-/U-Bahn, trams, and buses to get to either the Hbf or Ostbahnhof, whereas a railpass only covers the S-Bahn. On workdays, the pass would be usable anytime, whereas the Bayern-Ticket would only be valid after 9 am on a workday.
There is an additional day ticket to use to Salzburg - the Guten Tag ticket from Meridian. This ticket is also valid only after 9 am work day, and only includes the Meridian trains - from the Hbf or Ost. It's 2 euro cheaper at 35€ for three. If you are within walking distance of Hbf or Ost, why not? But if you need local transit to get to the station, the Bayern-Ticket is a better deal.