The 390 is a version of the 380, both based on a Morgan design. Catalina also made a Morgan 38 center cockpit with the same hull. They were all built, side by side, in the the old Morgan plant in Largo, FL, now owned by Catalina. All three are decent, reliable boats. But note that they are not the fastest 38's out there. The 387 is a totally new design, and has almost nothing in common with the others. Catalina has stopped production on all of the boats based on the 380 hull.
I've got friends with the 380/390 and they all like it. I've sailed with the past Commodore of the association. Get the boat surveyed by a professional. If it looks ok to him and you think it's worth the price, then go ahead. Warren Elliot is the tech editor. You can get his email address from any copy of Mainsheet. If you buy it, then join the association.