How to calculate Velocity?

Just to add to above question, Team members are same, who are working on all these 3 boards. 

First of all, why 3 boards?

Second, what "velocity" is requested? What is (in your organization) perceived as velocity? Where do you need this metric for?

@Xander, thank you for the reply.

1 - Each board is related to each Release

2- I need this data to define the Capacity during Sprint Planning meeting. I calculate it a an Avg of last 3 sprints.

Why not encourage each team to calculate their velocity in terms of how much work they do between each product release?

I am not understanding your configuration in JIRA. So you have 1 project, that I would assume 1 backlog and then 3 different boards? 1 board for each release? 

I would have had 3 backlogs (by creating 3 fake none started sprints, then putting different user stories on each release backlog by adding some release tag on them).  then I would plan a unique sprint where I would pick up different user stories from the different backlogs/releases and the plan would be based on the average velocity of the team. In this way, you will have a unique board for the sprint with items from different releases in the same board. After all the board is to give full visibility to all team members on what is happening.. all in a unique place. 

Another idea is to use a unique board and unique backlog but in the board you use quick filters based in a release tag. 

In your case you have 3 boards.

The velocity should be calculated on the sum of user story points done for all boards.. or all releases. That is the capacity of done items the team can accomplish per sprint. Then using that velocity you can plan how many user story points your team can commit per sprint and distribute those points among the releases or boards.. 

1 - Each board is related to each Release

You know there is an Release management part of Jira which would enable you to distinguish between releases without having multiple boards, right?