We were confused because we were thinking of the Eclipse 3 as being the 3-
gal Eclipse that many of us have (way too small for goldfish). I think you will be fine with goldfish in a 40-
gal. I kept 3 goldfish in a 35-
gal (one fancy comet, and two fantails) for ten years with happy fish and no problems. The pleco will take a little time to outgrow that tank so in the meantime enjoy! That tank is plenty big to have a lot of pleasure out of, so don't worry. Once the pleco gets big find someone with a 90-
gal or up and give him over. That tank can handle a rubberlip or a bristlenose, or one of several other smaller plecos for algae. Yes, some goldfish get big but it takes many years for them to get there, as long as they are not koi.
When you say gravel or no gravel, you mean bare-bottom? I don't find that terribly attractive, so unless this is a breeding or growout tank I am all for gravel, and the gravel is crucial to establish a biofilter. You need all the biofilter you can get with goldfish. I don't recommend undergravel filters, as they get cruddy and you can't get to the debris.
Please don't be discouraged and DO enjoy your tank. As long as you monitor the water and move the fish out once they get huge you will be fine.