Add a line, is extra data mandatory to take?

I have 2 lines on my account, both of which have the included 1GB high speed data. if i add a third line, do I have to mandatorily take a 3 GB data plan on this new line and have to pay an extra $10 for it? I do know adding the third line is going to mean +$10 a month. However if the 3 GB data is mandatory then that would mean +$20 a month for the third line, is that right?