I've decided to wait until the last minute before telling my roommate that I'm moving out with the third roommate.
That's a pretty horrible way to treat your roommate.
That's what I want.
It is the right thing to give ample notice to a roommate if you are planning to move.  It isn't nice to wait until the last moment.
Nah I'm going to do it anyway. I've been planning this for months.
What are you thinking? How would you feel if the situation were reversed? You could be inflicting a difficult financial burden on your roommate by leaving them with no time to prepare for having their rent triple overnight.