Costco always does group/panel interviews for most positions. It was a similar one for me with 3 people in the panel. They asked very very generic questions from a questionnaire that they have (like describe a challenging situation at your previous job, describe your most important achievement, etc.). There weren't any technical questions asked but I guess they let candidates speak about their technical abilities and assess them. It was very hard to understand the feedback or opinion they have on you and the process will take a long time (to schedule an interview, go through processes,etc). Be prepared with a clear list (bullet points) of all your strengths and achievements and do the best you can. It really depends on the panel and if at least 2-3 from the panel have a favorable opinion you would get an offer. But to end it, Costco is a great company to work for and has a very good work culture. They are one of washington's top rated employers for employee satisfaction.