I don't add career custom outfit. But sometimes when i've to plan outfit my sims, the career outfit appears, when i checked, it is too much more than 3. So i delete it one by one until career 1 remain...
It's not a glitch. Every time a sim uses the chemistry lab, they generate a new lab outfit. So yes, there will be one for every time they use it. The only thing you can do about it is to delete the extras.