I believe there is some sort of rule that carseats must be in window seats, so that in the case of evacuation, you aren't trying to climb over/around a carseat. But I could be totally misunderstanding the rule. Personally, I would do 1 kid per adult, assuming your mom is helpful. DC1 by gramma, DC2 with you so that you can nurse easier. Chances are, unless your flight is full, no one will want to finish "filling" your row, and you may have all 3 across even though its just you and DC....
--Mimi
Mom to Lala (2004), Bonus Mom to Big Sis 1 (1991) and Big Sis 2 (1992)
Grammy to Big Kindy Kid (2011), Big Pre-K Kid (2012),
Grandbaby Appendage (2014), and New Baby Grandboy (summer 2017)