I just found a quicker, easier way to switch your team for an event. It makes more sense, because I thought you should really be able to set your team BEFORE you ride in an event/race, rather than having to do it on the results page, which is what I described above.
So, if you click on your Profile on ZP, then click on Signups, you get a list of the events/races you have signed up to. Click on the event you've signed up for and want to change your team for, ie in your case click on your next 3R event and the same Settings option appears and you can change your team for the 3R series as I described above.
In summary, you can set your team when you have signed up to the first event in a League series, or you can change it via any event Result screen in a League series.
Hope that's clear!