No, that is not legal. First, there are no 3 day notices under Wisconsin law. Even then, the tenants are entitled to stay there until the end of the lease term, which is the end of September.
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No, this is not legal. Your parents cannot be forcibly removed from the property without court process. Prior to seeking court intervention the landlord must take certain steps, giving notice to your parents. If the landlord elects to terminate the lease without cause (i.e. assuming your parents have not violated the lease), he or she must give at least 28 days notice and the termination is effective only at the end of the rental period. In a one year lease, that is the end of the year long term.
Based on a one year lease, if your parents somehow violated the lease (for example - did not pay rent), the landlord would be required to give a written 5 day notice. Your parents would then have an opportunity to cure the violation during those 5 days. If they failed to cure, the landlord could go to court to seek an eviction at the end of the five days. If they cured the violation, but but violated the same term again within 12 months, the landlord could give a 14 day notice terminating tenancy.