My biggest concern when buying any phone is if it supports the radio bands for my cell service provider. This is usually a mess to figure out--the easiest method might be to use a website which has a database of phones and of service providers, but one phone model might be represented by six different variants and service provider band numbers and protocols are given without any context.
Anyway, the /e/os site doesn't say, but the fairphone website says the fairphone 3 supports these LTE bands: 1, 2, 3, 4, 5, 7, 8, 13, 20, 26
Here are my notes for some US providers, then Europe and China (the most used bands in parentheses):
T-Mobile
2 4 5 12 66 71 (2 4 12 71)
Sprint
25 26 41 (25)
Verizon
2 4 5 13 66 (13)
AT&T
2 4 5 12 14 17 29 30 66 (12 17)
Fi
2 4 12 25 26 41
Europe:
3 7 20
China Mobile (largest)
39 40 41
China Mobile HK
1 3 7 40
China Unicom (second largest)
3 8 40 41
Main bands
40 41