This question is unclear. If you mean 3 full electron shells and nothing else, then it would be argon. But every element after argon has 3 full electron shells also, but they also have other shells with electrons. If you mean a full 3rd energy shell such as in having 3d10 electrons, then it would be Zn but Zn also has 4s2 electrons. So, the question is a vague one.
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