Adding Additional Lines Later?

I was interested in signing up for a $50/mo Simple Choice Plan and use my own unlocked phone.

Later down the road (in the next 2-5 months), I would like to add 2 more additional lines for $30/mo, and $10/mo respectively for a total of $90/mo for all three lines.

Would I be able to sign up for an individual line plan now ($50), and simply add two more lines ($30, $10)?  Or would I have to initially sign up under a Simple Choice "Family" plan?