CP = (base Atk + Atk IV)*CPM * SqRoot( (base Def + Def IV)*CPM ) * SqRoot( (base Stm + Stm IV)*CPM ) /10
[EDIT: oops. Divided by 10 at the end.]
Attack has a bigger impact on CP than either defense or stamina.
Shuckle specifically has massive base defense stat and tiny base attack and base stamina. Attack IV will therefore have a bigger effect on Shuckle's CP.