∵ Take AB, BC and CA sides of △ABC as one side, and make square ABDE, BCMN and CAFG outside △ABC.

∴S 1=S2=S3=？ .

So choose a.

Comments: This question examines the knowledge of right triangle and triangle area. The key to solving the problem is to understand the relationship between the three sides of a triangle and the side length of a square.