Lord Ganesha, Goddess Lakshmi and Goddess Saraswathi are supposed to be the incarnations of Goddess Durga. Goddess Saraswathi is the Goddess of knowledge, Goddess Lakshmi is the Goddess of wealth and Lode Ganesha is the God to see that you will not face any obstacles in your journey of life. For any individual,l knowledge, wealth and intelligence are required for success. The intelligence and the way of overcoming obstacles will be given God Ganesha, The Knowledge will be given the Goddess Saraswathi and The wealth will be given by Goddess Lakshmi. So it is customary to worship these three together for success in our life. That is why we get a number of photos showing three of them.
A small story was there about the reason for worshipping God Ganesha and Goddess Lakshmi together. One day Goddess Lakshmi was feeling very proud because many people worship her for wealth and prosperity. She has gone to Lord Vishnu and started praising herself telling that she only can make people rich etc.. Lord Vishnu understood her mood. He told her that you are great but no lady is complete without children. Goddess Lakshmi has no children. She started thinking about this and finally gone to Goddess Parvathi to ask for a son out of her two sons. she explained her the position requested for a child. Paravathi after thinking a lot decided to give God Ganesha for Lakshmi. That is why people started worshipping these two together,