The northern corner of your house is related to wealth. So, Lakshmi puja should be done in that direction. Besides, you shouldn't keep more than two idols of the same God in your temple.
The idol of Lord Ganesha should be placed on the left side of Goddess Lakshmi, while Goddess Saraswati should be placed on the right side.All these idols should be in the sitting position. Keep them in the north-east direction and a picture of water or kalash should be there in north or east side of the idols. These idols should not face each other.