The Fairphone 2 did not connect to Verizon at all as they use different frequency bands than in Europe - and Fairphone is only selling to the European market. So far, Fairphone 2 users appeared to have the best results with AT&T. See also this forum topic: Taking Fairphone 2 to USA.
Using https://www.frequencycheck.com/carriers/verizon-wireless-united-states as source, it appears the Fairphone 3 does support the frequency bands used by Verizon, but there is a huge warning on that page that only approved devices can be used. The Fairphone 3 will probably not get approval, as it does not have an FCC certification.