Selectivity estimation is crucial in a query optimizer for choosing a good execution plan for a given query. Selectivity estimates of spatial joins can be used as responses to the specialized user queries that seek approximate figures. Spatial join selectivity can also be used for evaluating the correlation between datasets. With the proof that mutually overlapped rectangles are sharing a common region which is also a rectangle, this paper gives a selectivity estimation technique for 3-way clique intersect joins. The efficiency of our algorithm has been validated by us using synthesized and real-world datasets.