Best way to add a line transitioning from One Plus to Magenta Max?

I currently have 2 lines on the One Plus Promo ($100/mo) and I need to add another line, which would be +$45.

Magenta Max is currently $140 for 3 lines (since the third line is free), so that seems like a better deal since I'll be eligible for all the promos I can't get now.

Does it matter which order I do this in, adding a line and switching to Magenta Max? Does it impact the third line free? Can I get a device (iPhone) promo as part of adding the third line on Magenta Max?

The Tmo website is clear as mud.