We transitioned our toddler to her "big girl" room recently (she'll be 2 next month). We didn't want to move her after the babies came, because we didn't want her to feel like she was being "replaced" by the babies (her nursery will become their room when they are ready to sleep in it). I'm thinking maybe if you give Nathaniel his own "big boy" room now and make him feel special, it might be easier on him than after the babies? Then later you can split-up the kids based on sexes or however you decide to do it. Just my thoughts...there's probably not a "right" answer
Our DD slept in our bed until 9 months, and then she moved to her own room. With the twins I plan to side-car a crib onto our bed and have them both sleep there in the beginning.