San duo means three abundances: many  sons. happiness/wealth and longevity.  In the above motif, pomegranate with its many seeds represents fertility, peach represents longevity. Happiness/wealth is represented by the bats (fu) and  finger citron (which sounds like fu shou).

This motif originated from an episode related in an ancient text, (《庄子外篇》《天地篇》) allegedly written by Zhuangzhi.

Yao ( 尧) was a legendary Chinese ruler, one of the Three Sovereigns and the Five Emperors. Once, he visited Huafeng (华封) in present day Shanxi (陕西）.  The people of Huafeng wished him many male offspring, weath and longevity.  ["尧观乎华华封人曰:'嘻圣人。请祝圣人使圣人寿。'尧曰:'辞。''使圣人富。'尧曰:'辞。''使圣人多男子。'尧曰:'辞。'封人曰:'寿﹑富﹑多男子人之所欲也女独不欲何邪?'尧曰:'多男子则多惧富则多事寿则多辱。是三者非所以养德也故辞。'"].  It was termed the 'three congratulations of Huafeng (华封三祝).  He commented that more male offspring was to be feared, wealth meant more trouble and longevity invited humiliation.  They were not conducive to cultivation of virtue.