If I buy the 3 year 1 PC will it cover any new Interfaces etc and what people buy in 2015 or will I be stuck with the 2014 version for 3 years
In guessing the 2014 IS is more than adequate rather than paying more for the Premier?
You will not be stuck at all.
During the license period you will be able to install new(er) version, that is part of what you have paid for.
If IS is enough or if you want to get premier, is entirely up to you.
Comparison:
http://www.avast.com/en-us/compare-antivirus