A single trip within one zone cost 2,60€. A single trip within one ring costs 1.25€ (one strip @ 12.50 for 10 strips).
No, a trip (other than a Kurzstrecke or short trip) within a ring requires 2 strips (2,50€), as does any trip within the zone. A Kurzstrecke is 2 stops on the U-/S-bahn, 4 stops on the tram or bus. So, you could go from the Hauptbahnhof to Marienplatz on the S-Bahn for 1,25€ using strip tickets whereas it would take a one zone ticket for 2,60 using a zone ticket.
A Kurzstrecke is independent of zone borders. A Kurzstrecke would allow you to go from Allach, inside the inner zone, to Dachau, in the second zone, two stops away on the S2, for 1 strip, or 1.25€, vs 5,20€ using a 2 zone, single trip ticket. For someone living in Munich and having year pass for the inner zone, going to Dachau would require just 1 strip for Karlsfeld to Dachau.
In your case a three-day inner zone Tageskarte would cost 15€ and cover your travel in the inner zone. Four strip tickets would cost 1,25€ each and cover you from Karlsfeld to Dachau and back, and from Feldmoching to Oberschleißheim and back, but buying 10 strips to get 4 is not economical. You might be able to buy individual strips for 1,30€ each. Check at a ticket counter. If so, that way would cost 20,20€. Or, a weekly IsarCard for rings 1-6 costs 25,70€ and covers travel to Dachau and Oberscheisheim, but 2 XXL and 1 inner zone Tageskarten (since on the third day you won't go out of the inner zone) would only cost 22,20€.
The price page for MVV tickets is at www.mvv-muenchen.de/de/tickets-preise/preise/index.html#c9888 (in German). Innenraum is inner zone, 3 Tage is 3 days, München XXL is for the the two inner zones of Munich, for rings, Wochenkarte is the one week card.