What are you hoping for?

• You have a lease obligating you to pay a set amount each month for three years,
• By both your own admission and the records of your landlord, you have not paid in full, and
• While you attempted to sublet or assign your lease, you were not able to do so.

I'm not sure what notice you could have given in the middle of your lease that would possibly have excused you from the remainder of your term without the consent of your landlord. Term leases are enforceable in each province, and the tenant generally needs to work with the landlord to end them early. Without a written agreement to terminate the lease, I don't think you're going to get very far.

On your narrative, you very likely owe the outstanding rent. It'd be nice if your landlord negotiated with you, but the debt appears valid and you seem to be set on avoiding it so far. Sending the debt to a collections agency is a legal way to attempt to collect on it. If you disagree, you can refuse to pay, and it will stay on your credit record for a long time. You could dispute that, which will likely precipitate a small claims suit by the collector - and I don't see you winning that suit.