If you get a Hero for your line, and want to keep all 3 lines on one Share plan, you'll have to do an Everything Data. The 1500 minutes is $129.99 + $19.99 for the 3rd line (or about $150 a month), and the 3000 minutes is $169.99 + $19.99 (about $190) for the 3rd line. You can put your line on an individual plan, and the other 2 on a share with just text, but you'd be looking at $69.99 for the Everything Data 450 plus $99.99 for the Everything Messaing Share 1500 for the boys (so $170). You can block data to their phones on the Everything Data Share plans, but it's still included in the plan cost (they don't take any money off the bill).
And if you're planning on the Rumor 2 because they're free, you have until April 17 before pricing changes again. After that, it may be different phones that are free, but it's almost guaranteed there will be *SOMETHING* with a full QWERTY keyboard for free, either with the "Web Special" discount or after a Mail-In Rebate.