Pure Asterisk solves this problem in a different way. Unfortunately we get into an area of terminology which causes a lot of confusion as extension means something different in Asterisk from its meaning in FreePBX.
In traditional telephony, there is the concept of an equipment number and a directory number. What you want to call to contact these three people is actually a directory number, which Asterisk calls an extension. However FreePBX calls something closer to the equipment number the extension, and then generally confuses the directory and equipment numbers. I think it uses the term virtual extension for the case where a directory number doesn’t correspond to an equipment number.
So, with pure Asterisk, you can have different equipment numbers that are associated into a single directory number by using a special syntax for the Dial application arguments. Also, with pure Asterisk, there is no automatic means of making a call, from outside, to a particular equipment number, there has to be explicit provision, in the dialplan, to map the directory number to one or more equipment numbers. (In fact best security practice is to make directory numbers and equipment numbers different, although that is not commonly followed, and FreePBX is based on not doing that.)
SIP multiple registration further complicates this by doing a similar split with the equipment number, splitting it into an address of record, which is closer to a directory number and a contact address, which is closer to an equipment number.
However, although, with pjsip, this mechanism can also be used parallel call multiple devices, the split into multiple devices is less explicit, with the result that I’m seeing people asking about how to send calls to just one of the devices.