Not all landlords are willing to do 3 months -- regardless of whether they do a lease or tenant at will agreement. Most landlords want long term tenants and there is work to be done after a tenant moves out regardless of if they have been there for 3 months or 3 years. So, you have to find a landlord that is willing to do this. If you are looking for June rental, then the apartment will be available again for September 1, so that might be ok for some landlords.
Unless you are willing to pay the full commission to a Realtor, your best bet is to find landlords who are trying to rent the units themselves. You may also want to look around to see if someone is willing to do a 3 month sublet. That might be your best solution.