Well, when are you heading out? Have you owned a Catalina before?
The 390 was never one of their more popular models. I honestly do not know a lot about it. Thus, I will speak in general terms:
1) The watermaker will probably need maintenance. Who is the make? You better just plan on a rebuild even if he pickled.
2) Check your sails. 3 years of cruising is like 10 years of typical use.
3) The only issue with siting that long might be the engines. Diesels do not like to sit. The hours do not bother me, but how were they put on? Was it 300 hours/year? That is high but fine. Or, was it 500-600 hours the first threee years then nothing? That would not be good. Of course, there is no way to know. Given the hours, etc, get a engine survey. This is NOT part of the regular survey.
will probably be fine. It will probably be out of date, but who cares. The only difference is it may not have an overlay, but that is not the end of the world.
5) Go join Mainsheet and get to know the folks taht run the 380/387/390. That is your forum group. You can go to catalinayachts.com and look under mainsheet for the Tech editor. I forget his name. Seems competent. Still, I do not recall Catalina selling a lot of those. I think it is a modified 380, as I recall. Check with the TE for Mainsheet.
Hope that helps. If you have specifics, write me or write back here. If it is a modified 380, I can be of a lot of help. I lived/cruised on one with the fam for several years. Now doing it on a 400. I am the TE for Mainsheet for the 400.