I do get the odd appt cancelled. But I have never, ever had to wait 3 months for it to be re-scheduled! Normally, within a week or two.
Generally, they are cancelled because derm has a meeting or conference somewhere. Fair enough.
I suggest making a phone call and saying, although you appreciate the derm is busy, waiting 3 months for the next one is a little long and could they possibly manage to fit you in sooner...particualarly as you are flaring and need to see him/her.
SJ