you could do 2 lines with 200mb each data, and a 3rd line with no data, 1000 minutes shared, for $65 a month on t-mobile's value plan. note that this requires you to buy your own device and still has a 2 year contract. you can also bump up to 2gb data for an additional $5 on the data lines.
on att, you could get 700 mins/month, 2 smartphones with 300mb data each, and a 3rd dumbphone, for $120 a month.
or att's new "mobile share" it would be $40 base, $45 for each smartphone, $30 for a basic phone = $160 for 1gb of shared data, unlimitd voice/text
as you can see tmobile is by far the best value. if you get non-LTE phones, tmo's HSPA+ is much faster than ATT's as well. the only question is if their coverage is sufficient for your needs. in my experience, it's fine in most big cities, but drops to 2g as you get a bit further away. often only 2g on interstates as well. check the coverage maps to help you out. also when purchasing smartphones for use on tmobile you need to make sure they support the aws bands for hspa+ (1700/2100mhz)
even if you estimate $400 up front for the two smartphones, and $100 for a new dumbphone, you're still saving $55 a month for 24 months = 1320.
if you assume that the att smartphones would cost $100 each, and the dumbphone was free, you've saved $620 over 2 years.
if the nexus 4 is back in stock on google play by the time you're ready to switch in June, you should definitely give it a look for use on tmobile.