For VIVIFI, If i share it with 3 people, does that mean each of us will have to pay an additional $5 for a sim card? Does that mean all of us will be using same phone number?

Not quite. The share plan is $30/month ( 40GB/300min) on this plan you can add additional SIMs for $5.35 each ( get addl 2GB) and so on.

So for 3 people, it should cost you $40 odd and you will have 44GB to share amongst the 3 of you.
In short 3 SIMS for 3 People