GPS + GLONASS or GPS + Galileo or only GPS ?

There is no absoluty answer. You need to try each settings.

For me (in switzerland) the best is only GPS

there is someone from United Kingdom here who checked ?

It doesn't matter where it's tested. What seb2020 is saying is there are too many factors for any given location. Weather, satellite location, bldgs, etc.  What combination works best now may not be the case 2 1/2 days later running the same route. 

Okay so where are you from and what’s set up you use?

I'm from the Pacific Northwest. Normally, I use GPS Only. Other times I need the addition of GLONASS.  I've never noticed any improvement using Galileo, but I've never had the need to test it in depth. 

Okay so USA far away from Europe so signal gonna be completely different than Europe and UK plus in your area elevation is much bigger so this help for the GPS signal aswell because you are much higher above the sea that means signal will be stronger so this will be completely different story for you. I need experience from someone from UK this will be the best advice for me anyway thank you for your help 

every country is different for GPS signal and set up.

Okay so USA far away from Europe so signal gonna be completely different than Europe and UK

I'm sorry but where this kind of strange information come from ?
I'm curious, really.
I thought the satellite were in orbit far from any longitude issue... If you are in California or in the UK what would be the difference ?

I don't find any here:
https://en.wikipedia.org/wiki/Global_Positioning_System

Unless you were talking about CORS Network ?

"Many countries have a CORS network which covers the entire nation, allowing accurate GPS positioning anywhere in their country. CORS usually also contribute data to global observations that make the GPS system more reliable and accurate. They also provide data for scientific studies such as plate tectonics and meteorology. To be useful for the study of tectonics, the permanent marks used for the CORS stations must be geologically stable, and the observations should be continuous and for many years.

Australia has state based CORS networks covering the most populated areas. These are generally commercially run providing real-time access to subscribers.

Australia also has a highly accurate freely available national CORS network. The Australian Regional GPS Network (ARGN) may be used with the on-line processing system AUSPOS. This allows GPS data from a survey quality GPS receiver to be submitted via the Internet and a calculated position to be emailed back, typically within a few hours. The calculations used to produce these positions use more accurate satellite orbits and within about 24 hours of observation can give a position anywhere in Australia with an uncertainty of a few centimetres."

https://www.gpsworld.com/esa-to-use-cors-networks-for-global-error-mapping/

Is that what you meant ?
But our Fenix don't use any CORS Network...

But every country have a different elevation and structure of the land plus not all countries in the world have the same strength of signal some countries have poorer signal quality and some countries do better there are many factors which affect it GPS signal.