Start at page one of the Vicidial Manager's Manual (available free at EFLO.net) and don't skip any of the exercises. Eventually you'll get to the point that you can do this without asking for help. (IF you don't skip anything or jump around, since some of the exercises are interdependent ...)
That being said:
DID and Extension are not related to each other in Vicidial.
* A DID is an inbound phone number, upon which calls arrive.
* An extension is a Phone in Admin->Phones and those have VOIP phones registered to them and are on agents desks.
An inbound call ordinarily works like this:
1) Arrives on a DID ... which
2) Routes to an Ingroup (ONE ingroup, not three) ... which
3) Is "allowed" in any number of Campaigns (The campaigns would need several configuration changes to allow inbound)
So you can have three DIDs which route to three different Ingroups which are all allowed on as many (or few) campaigns as you would like.
For agents to receive calls, they would log in to the campaign AND select those ingroups to receive "blended" inbound and outbound in their phones.
For all of this to work properly, however, you'll want to take my advice and start at page one of the manual. When you get to the end all of this will work. If you hit a road-block, bring us the page/line/version of the manual and your problem (what you expected to happen, what really happened, and the relevent configuration options ... perhaps some debug output) and someone will walk you through to the next step. Many don't hit that snag and it "just works", in which case you could just work through it and get where you need to be without any further delays. Which is why the Manager's Manual exists.
Happy Hunting!