Hey there, according to the API you are missing the third out of the seven chests, which is actually the 'first chest' mentioned in the first catalyst quest. This chest is referred to as 'Gatekeeping' in your Triumphs, 'find and destroy the Vex Data pods'. This chest can be found at the second conflux, the one before the final boss. The activity is the portal that is straight ahead of you, when you first enter the transit portal room. You've already done #5 which is to the left, and #7 which is to the right. If you go there and do that, then it shoud complete the first catalyst quest. This can be done on Normal if you want, it doesn't need to be done on Expert. Since then you've met the conditions for catalyst quest 2 (chest #5) and quest 3 (chest #7), you'll be given all three catalysts when you hand the first quest in to Banshee.