Note 10 BOGO Promo Question

I have two lines with T-Mobile 55+ company and I want to get the Note 10 plus and get a note 10 for free, but they are telling me I have to add a new line (so I'd be adding a third line in order to qualify to buy a note 10 plus and get a second note 10 free, so here's what I am thinking - if I add a third line and do the promo to buy a note 10 plus and add a third get a note 10 for free , but since I only need 2 lines I want to cancel the second line I currently have that is not part of the purchase or the new line I'm adding, it's a line I've had for over a year that does not have a contract or phone debt and if I add a third line to qualify for this promo I won't need my second line, would this effect my promotion deal? T-Mobile rep says yes and that I have to wait 90 days before cancelling in order to not effect the promo, but I don't understand why that would matter if my first line is buying the initial note 10 plus and a third line is being added to qualify for the promo (the promo says buy a note 10 plus and add a new line to qualify for a free note 10 or $950 in credits towards a new note 10 plus) so why would my second line that has nothing to do with my purchase of the new line credits effect my promo ? I should be able to cancel it after the promo without effecting my deal right? I was told if I cancel after 90 days it won't effect my promo but why would that matter if I cancel in one day or 90 days? The promo says you can't cancel the new line you added, or the first line which initiated the purchase of the note 10 plus, I don't see why it would effect anything, please advise 😥 I feel like I'm being forced to switch to another carrier just to switch to lines and qualify for the promo without having to keep a third line, please help