Your translation is almost correct. One never says "τη εβδομάδα" but "τη βδομάδα" (you have to omit the "ε"). It's the same as "μέρα" and "ημέρα" (= day). :-)

Τρία για βδομάδα just makes no sense in Greek, τρία για μία βδομάδα means "three for a week" (in the sense of: How many shirts do you need for the trip? I need three for a week). Three a week can be Τρία την βδομάδα or τρία ανά βδομάδα.