Airlines hold back seats for just this reason,,,, to seat families together as much as possible
They hold certain seats for what they call " under airport control"
I would try to change seats during online check in and if that doesn't work, get to the airport early
Talk nicely and rationally to the agents at the airport and they will do their best to get you 3 seats together.
Alaska hold back the last 2 rows most often for airport control.
You probably won't like the last row because of the limited recline and your nausea issues