What carrier/plan is best for me?

Hello. Our recent 2 year contract with AT&T has expired and we are wondering how to proceed to get the best value with our mobile phones. We have 3 lines (all smartphones).

I saw an article which I figured would come upto 5 a month: cnet.com/news/sprint-subscribers-can-now-upgrade-to-latest-iphone-model-anytime/?tag=nl.e703&s_cid=e703&ttag=e703&ftag=CAD090e536

If we only want about 3-5 GB Shared Data (3 smartphones [2 iPhones and 1 android]), what would you guys suggest to do? We'll look at any carrier and any plan. We dont upgrade our phones and do it approx every 2 years.

Thanks.