I would be glad to answer any questions you have on FHA Financing. A 3-4 unit you can use 75% of rental income from the other units to help qualify. The property will also have to pass the self sufficiency rule.
Self Sufficiency Rule; The maximum mortgage amount for 3-4 unit properties is limited, so that the ratio of the monthly mortgage payment, divided by the monthly net rental income does not exceed 100%, regardless of the occupancy status. This is also taking into consideration, a 25% vacancy factor.