You can use PJSIP to have an extension on multiple endpoints, but this will not be a true shared line appearance. Once the call is answered at endpoint 1, endpoints 2 or 3 will not have visibility into it. endpoint 1 would have to park the call and then endpoint 2 or 3 could pickup from that parking lot. This is a very requested feature, but not something native to FreePBX.
Options?
Asterisk supports share line appearances if you go that way: Shared Line Appearances (asteriskdocs.org)
@PitzKey has an awesome writeup on his blog about “stealing” calls, in my opinion this could be the foundation to making something that behaves like a normal shared line appearance. But this will require work. PitzKey Is A Theif! - FreePBX / Tips and Tricks - FreePBX Community Forums
If you do make progress in this space, post a writeup. There are so many who desire a SLA, I am sure it would read well.