5 star C-3PO Gear

You'll want Chirpa's zeta eventually anyways for when you go after 7* 3PO. Also, when you go after 7*, use Wicket instead of scout. Makes the event easier but also a 7* wicket gives you a few extra zeta's every month.
As far as gear for a 5* 3PO using 5* ewoks, I would guess 8 or 9 would probably do the job. Even if you need a little more, that's not the end of the world since you'll need it eventually. I should also point out that a Padme team will take you farther in squad arena than a JKR team. This is coming from someone who uses JKR and is trying to skip the Padme team in arena and is struggling currently as a result.
I would aim for g7 on everyone and just start going after the event. You can retry as much as you want and don't have to wait for it to come back around so there's no reason to not start trying earlier than when you think you'll be ready. Most people who have gotten 3PO at 7* did so by trying over and over again until things fell just right for them. This event does rely on RNG a lot, specifically the lower gear and mods you have. Follow the guide below for further advice on how to do the event. Do not underestimate the importance of mods for this and similar events.
https://wiki.swgoh.help/wiki/A_Quickish_Guide_To_Murder_Bears