Live clam to canned conversion

by Frank
(San Francisco)

I have a recipe for clam sauce that calls for three cans of chopped clams. I've always made it with canned clams, but I would like to try fresh whole clams.
My questions is how many pounds of fresh whole clams would I need to buy for this recipe. Do you have any "rules-of-thumb" when it comes to converting fresh to canned and vis versa?

Thanks,
Frank