I refunded the amount for the entire 3 hours.  As for the reason I had to leave.  My dog was having seizures, I was called by my dog sitter and needless to say, I did not think it wise to remain for the remainder of the appoinment (the other two hours) and then the drive back to Guelph (another 1 1/2 hours).  The reason for the previous cancellations were the same thing.  I had been assured by my vet, before both rebookings that they had remedied the problem.  It turned out not to be the case.  Before the last rebooking I had offered to refund the pre payment.  But we decided to see what the vet had to say.  They believed the seizures to be caused by a medication she had been given, and blood test revealed it was no longer in her system.  Therefore I felt safe to rebook, having assumed my vet knew what they were talking about.  It turned out their diagnosis was incorrect, which lead to the new seizure on Monday afternoon.
I am generally very reliable, but my pets are my children, anyone who has pets will understand this thought process.  I feared that if I did not return immediately, that perhaps she would have passed on before I ever saw her again.
I believe I acted in the best manner possible given the circumstances, and of course any future appoinment would be at a discount as well.  Looking back on it, it might have been best to wait, but, I am not a vet, and could only act acccording to what I was told.
*kisses*
Kassie