3 kg of banana 5 yuan, then 5/3≈ 1.67 yuan/kg, that is, per kg of banana 1.67 yuan;

4 kg of orange 6 yuan, then 6/4= 1.5 yuan/kg, that is, each kg of orange 1.5 yuan;

The 7 yuan of apples is 5 kg, so 7/5= 1.4 yuan/kg, that is, apples per kg 1.4 yuan;

So apples are the cheapest, per kilogram 1.4 yuan.