Considering all the comments I’ve seen about their phone basically falling apart it kind of sounds like you should just get an iPhone SE and factor in a battery replacement after 3 years. Day one updates for 5 years and I am severely doubtful you’ll have as many hardware problems if you don’t damage it. Better camera, too.
The Fairphone 2 actually had 7 years of security updates before the support was withdrawn, so it's pretty likely the Fairphone 3 will last at least 7 years, maybe even longer.