Just for reference:
In 3 years of living poly with husband and boyfriend in household-
We've only managed in the last 5 months to go longer than a month without a freaking dramatic HUGE fallout.
It's taken THAT LONG to settle into some sort of reasonable emotional stability and routine!
Furthermore, with EACH new addition to a dynamic there WILL be change, stress and headaches (consider what happens with the birth of children, weddings, deaths, job changes etc). It's part of the reality of life. Each individual is unique and bring their own changes to the dynamic, so getting used to ONE person or ONE type of situation doesn't mean that there won't be more struggles or issues to face later.
It also doesn't mean all is lost and there's no point in trying.
"Love As Thou Wilt"