In Threes Company he cooked for the girls and seemed like a chef then, but come Threes a Crowd, he hired this E.Z guy to cook for him at his Bistro. All Jack mostly did after that, was dress up nice and welcome his customers. I mean did he suddenly lose his skills as a chef? Most Chef's I've heard of, that own their own restaurants, insist on doing the cooking.

We did see him graduate cooking school but I guess with the bistro he wanted to focus on customer service while he had a helper like E.Z. around.

But there were a few instances in which we do see Jack in the kitchen prepping things, for example I'm assuming Jack did the cooking in the episode that has E.Z. going out with the sorority girl.