Sorry to hear about your loss.
Gold nuggets are well known to be very aggresive towards each other and will battle to the death over territory. If this is your newest fish It is very possile that one or both of your other two have killed it
extract from www.planetcatfish.com
Gold nuggets are territorial to other bottom dwellers and downright vicious to other gold nuggets. This stems from a desire to find and defend the best spawning cave, so take this into account if you plan to introduce these fish into a catfish tank. They do well in a community environment that fits their other requirements. Be careful with adult fish, they can kill each other in a crowded space.