what's left out of this thread is that if one ports in a line onto a new account, one is eligible for a 20% discount on tmobile one services for life. it's called t-mobile's insider hookup. you get a code from an employee and enter it online.
it makes the 3 lines for $100 with kickback about $64 and no taxes/fees. Heck, 5 lines would be an additional 40, but perhaps get another $20 ($10 * 2 kickbak) and with the 20% discount, comes out to 5 lines for $80 if don't use a lot of data, and if you do use a lot of data on some lines, it only goes up by $8 a line (i.e. still get 20% off the extra $10 you're spending). And if there's only one line that uses a lot of data, make it the free one as not elligible for the kickback anyways.
Basically, its very possible to be spending less than $20 a line with what they were offering this weekend.