2-3 night shifts/month is not a 50/50 rotation. So either the manager is not being up-front with you, or the staffing does not currently require a full 50/50 rotation. What will happen if staffing changes, and you are required to actually work a full 50% of your time on each shift?
I did a 50/50 day/night rotation, and it was the worst 9 months of my life. When I worked days, I left for work at 6am, got home at 8pm, worked in a unit with no windows, and for 3-5 days in a row, never saw the sun. When I worked nights, I left for work at 6pm, got home at 8am, struggled to sleep in the daylight, and could barely manage to function for my 3-5 night stretch. The constant rotation between days and nights was hard on my sleep habits, eating habits, personal life and health. I finally went to my manager and told her to put my on a permanent shift (any permanent shift was fine with me), or I would quit.
I would never agree to do a 50/50 rotation again.