Slightly off-topic, but how are you getting a 3 planet home system? I don't do ARCs because I consider it "cheating", but from what I understand you're working with all the default home systems and just using them with different or custom races, and none of the defaults have 3 habitables.
You get the three planets simply by picking a home planet in the system that's normally a PQ0. For example by picking Mercury as your starting home planet when playing the Terrans you start on Mercury which will be a PQ10 and you'll still have Earth as a PQ10 and Mars as a PQ4 thus you'll have three starting systems in your home planet. Same with most other races. For the Iconians just pick New Iconia II. The Korx are interesting because their secondary is a PQ8 and so by picking something like Korx V you get a PQ10 primary and 2 PQ8 secondaries. It's almost like 3 starting quality planets.
You can actually do this without even using an ARC just by changing the home planet in the edit screen of the race that you want to do this with and then next time through pick that race a voila, you have three planets in your home system. I actually do this will all my opponents as it essentially gives me 10 extra planets in the galaxy. There are a couple of races that this doesn't work with, IIRC the Krynn and Korath but that still leaves ten total systems to use for whatever race you play plus all your possible opponents.
As far as an ARC being "cheating" for one that's a pretty harsh word for something that is pretty innocuous, however if that's the way you feel about then you are of course free to not use it. Also this was fully and openly discussed within the community when it was discovered and even brought to Cari's attention (see https://forums.galciv2.com/172405 reply #21) and it wasn't deemed significant enough to do something about and so I don't really see it as cheating. As far as exploits go I think ARC is pretty far down on the totem pole. If I had my druthers the ability to purchase buildings and upgrade ships while bankrupt would be something that I would be happy that doesn't exist however just because I'd rather things were different doesn't mean that I won't use a feature that is otherwise well known (and well used) that provides a benefit.
However there are many ways to play the game, modding, role play, etc. and score grinding is just one of those ways. No one should think less or more of someone because of the way that they choose to play. Of course there are things that are downright cheating like hex editing a save game file and in my opinion using an ARC to get three starting planets is a long way from that.
