[Between the Lines]

LOL. I don't know what to make of three ribs.

[Frapster]

I would speculate that visually the three ribs might be more identifiable than just one rib. So rather than gamble on one they chose three for its obvious Trinitarian connection as opposed to 2 or 4 or more. In the end it still symbolizes the new Adam and the birth of the Church which is consumated with the spilling of the blood (redemption) and water (adoption).