You are entitled to receive reasonable notice or pay in lieu of notice as this is a dismissal without just cause.
Generally, in determining what is reasonable notice Courts look at several factors including the length of time you worked for the employer, your age, your position, the likelihood of finding new employment etc.
At the high end, if you were in a managerial position, the Court would likely order one month's notice or pay in lieu of notice for each year of employment. If you were not in a managerial position the Court would order somewhat less.
So you what you have been offered is just over 9 weeks for three years and that may not be so bad. A court may not order more.
It is possible a court would consider ordering somewhat more because of how badly you were treated but that is never a sure thing.
What may make the most sense is to tell the you cannot sign a release without legal advice and take the release and see an employment lawyer face to face who may want to send them a letter demanding a couple more weeks pay.
You can contact the Law Society and use their Lawyer Referral Service. You will be given the name of a lawyer and can consult with the lawyer and the first half hour will be free.
The number is:
1-***-***-**** or(###) ###-####(within the GTA)