Actually as a customer of T-Mobile your figures are incorrect.
First line of service is $50 and $20 EXTRA for unlimited data w/500Mb of teathering per month.
Second line is $40 and $20 EXTRA for unlimited data w/500Mb of Teathering per month.
Third line is $30 and $20 EXTRA for unlimited data w/500Mb of teathering per month.
There are no subsidized phones so add $650 per line payable either all at once ($1950.00) or payable at lower payments over 24 months each device say ($25x3=$75) But their is tax on each device up front, so lets say at my tax rate in example 8% is $52 each line plus a down payment of say $100 per device, and T-Mobile charges $10 for each SIM card. so lets add this all up. $100+$52+10=$162 each line if you use EIP plan. or $1950+$30+$156 =$2136 due at joining.
But after you pay off everything its around $170 per month for three devices plus taxes and fees. or $225 per month.
Will be more if you add lines 4&5 on the plan. T-Mobile can give you a proper estimate.