H1B Visa Extension

Hi All,

I need help on H1B Visa extension, so please spare some time to read my query. Its really important for me.

My company A had filed H1B visa and I got it approved on Jan 13. I was going to travel and got my visa stamped on passport. But somehow due to some finance issues my travel from company A got cancelled. So I left company A and joined a new company B.

Now I am re-joining company A on May 15 as they are giving me good offer. Now my main question is: My h1b visa is getting expired on 30 Sep 2015.

So I want to know if company A can reuse this stamped visa which they had initiated if I need to travel? If yes then is there any process needed to re-transfer visa to company A or its not needed as stamped visa already has company A as petitioner ?

If my visa gets expired, can company A file for extension of 3 years? And does the extension has to be applied immediately after H1B visa expiry or it can be done at anytime?

Thank You,