What is the best Diono car seat model and why?

I have 3 year old twins and a 3rd child on the way. I want to purchase 3 Diono car seats. I’m confused by the different models, ie: R, RX, RXT, 3RXT, etc... what one is best for 3 year olds? And how about for a newborn? Also trying to fit all 3 across in my ‘08 Odyssey