“Farmers of low income” shall mean persons or families who at the time of their admission to occupancy in a dwelling of a housing authority: (1) live under unsafe or insanitary housing conditions; (2) derive their principal income from operating or working upon a farm; and (3) had an aggregate average annual net income for the three years preceding their admission that was less than the amount determined by the housing authority to be necessary, within its area of operation, to enable them, without financial assistance, to obtain decent, safe and sanitary housing without overcrowding.