Such as goldfish are cold water fish and huge waste producers. This one looks to be a comet which gets over a foot long and lives over 20+ years I believe
The fact that oscars are new world cichlids and have widely diffrent preferes parms then old world. Not to mention the fact they also get big. Like a 55 might pass for one oscar by itself. Let alone three these are at risk for hole in head disease also, since you have such big waste producers and that the main cause of that is poor water quality/ poor food. And if that gold fish was a feeder at one point, poor food is very likely as most feeders are riddled with parasites/ fat.
Angel fish are the more delicate of these new world cichlids and can form a pair. Pairs at absulte bullies for the most part and can bully the other one to death. Not to mention that they are at risk for infection from possible fin tearing.
African are complete in left field here, they prefer harder