My husband and I are visiting Paris for 3 days, is the decoteure card the best option and can I buy them both digitally and put them both on my phone?
The Navigo Decouverte week pass is likely your best option, assuming you arrive anywhere from Monday to Thursday. From Friday onwards, the Navigo Decouverte won't be active until Monday. (If you land Friday, a Paris Visite 3-day 5-zone ticket would be the simplest option, covering you thru till Sunday midnight, but not Monday daytime.)
Unfortunately only one Navigo Decouverte week pass can be put onto a mobile phone at a time. So each of you would need your own mobile phone with Ile-de-France Mobilites app installed.