After reading 3.0 notes and saying that $di is now in $this I wonder if it will be possible to use $this->session in models instead of $this->getDI()->getSession(). Need to upgrade and test next week, no time now :(
I'm guessing that you really didn't read 3.0 notes. It's saying that your service definition is bound to di(so functions are the methods in Phalcon\Di object, this is why you can use $this. Are your models are defined in di as service defnition ?