the price provided if stated 3 cleans per year at a set rate allows you 3 cleans at that price but this must be confirmed in writing. email confirmation covers you in court.
By law you have formed a legal agreement and in doing so you could reply with a stern reminder of this.
They may bow down.
I did this with a major hotel chain who thought it would be okay to get a just 1 clean and cancel when a 3 clean minimum was agreed before starting.
They paid me and I did the cleans.
Fair is fair.
They are in breech if they confirmed.
Good luck