The Fairphone OS on the FP3 is pretty close to stock Android. It has the usual Google services and apps installed, but no bloatware. Fairphone has made some modifications, for example their (proprietary) smart charging, which does not use Quick Charge for the last 10 percent of the battery capacity.
As of now, there is only one official alternative OS available for the FP3, namely /e/OS, which is a fork of LineageOS. Officially still in beta, btw.
The Linux distros which are currently available for phones are at best proofs of concept, but not suitable for average and every day use IMHO. More importantly, they’re lacking the rich app ecosystem that Android provides.