Your lease agreement with your tenant is a privately negotiated contract and you may negotiate any termination term that you and the tenant can reasonably agree upon. In determining an appropriate amount, it may help to calculate your maximum damages if the tenant simply defaulted.
The lease obligates your tenant to pay rent for the entire three year term, regardless of whether they live in the home or not. If they default on the lease, you would have a claim for damages. Unless there are contrary terms in your lease document, those damages would equal the total rent that would have been paid over the remaining two years, plus any costs of collections (and possibly attorneys fees), minus any rent you receive from a replacement tenant.
As the landlord, you are obligated to mitigate your damages in the event of a tenant default. This means that you must use good faith efforts to find a replacement tenant. Your current tenant is only liable for the gap between their default and the new tenant's occupancy, including any loss you suffer because you must rent the home for less money to the new tenant. For example, if rent is $1000/month, it takes you six months to find a new tenant, you spend $200 on advertising, and the new tenant only pays $900/month - your total damages equal $8,000 ($6000 lost rent during six month vacancy, $200 in advertising expense, $1,800 in lost rent based on lower payments from replacement tenant).
One option is to hold off on determining the final number until you have a replacement tenant in place. This would allow you to determine exactly how much you have lost as a result of the tenant moving out. On the other hand, your current tenant may want a quicker resolution. If so, you will need to make a business decision as to how much you must be paid in order to take on the risks associated with taking the house back. In the example above, some number close to $8,000 would be appropriate, however, that is based upon a series of assumptions that may or may not come to pass in your situation.