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Now..a yard of concrete is a cube 36" x 36" x 36".
So, you have (in inches) a slab that is 120" x 240" x 3.5"
Therefore you need (120x240x3.5) divided by 36x36x36 which is just over 2 yards (2.16 yards). If you order 2.5 yards, you can get almost 4" of thickness.
Let me know if you need further information.