My friend and I are travelling and we reached a location where qasr is allowed but they have started praying (Zuhr).

Do we join them? If we do, do we salama after two rakahs?

Also, which is preferable: praying qasr for the first three days or throughout our stay in the location?

Jazakumullah Khayran

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KingJB:
My friend and I are travelling and we reached a location where qasr is allowed but they have started praying (Zuhr).

you've not gotten to your destination right?

Do we join them? If we do, do we salama after two rakahs?

join them and complete the salaah because the prophet said “When you hear the iqaamah, then walk to the prayer, and you should be calm and dignified, and not rush. Whatever you catch up with, pray, and whatever you miss, complete it.”

Also, which is preferable: praying qasr for the first three days or throughout our stay in the location?

Jazakumullah Khayran

if you formed intention of staying for more than four days, then pray in full right from day 1.

and Allaah knows best.

Jazakumullah Khayran