Well, there are fly sizes that are certainly easier to cast with a 3wt, and ones that are harder. A big weighted streamer, for most casters, would be a bit more difficult than a #16 Adams. It wouldn't be nearly as pretty a cast, anyway. Conventional wisdom is great, but it doesn't mean you HAVE to do what "everyone" says.

I used to fish a spot that had lots of 18" fish in it and a 6 or 8# fish wasn't that hard to find if you kept your eyes open and paid attention. Good hatches, reasonably small water, perfect (IMO) for a 3wt. One day I was fishing it with the 3, and after releasing one of the big ones, had a gentlemen approach me and criticize me for using too light of a rod on the fish. Guess he saw it bent into the cork or something, as I hadn't fought the fish that long before landing it. Shortly thereafter, I hooked another similar fish, and the next gentleman berated me for not "playing" my fish long enough. I pointed to the first gentleman and suggested that he discuss it with him, not me.

I get twitchy when the fish (trout or steelhead for me) go into double digits, poundwise, with a 3wt in my hand. More because I am not used to fighting fish with the first two feet of the rod rather than the middle. Given room and decent tippet and a skilled person holding the rod, it can be done, and regardless of what you may hear, it does not necessarily mean the death of a fish through an overly long fight (again, the skill of the angler is the limiting factor). For a 20' wide stream, throwing a #16 dry fly, and the average trout (ten to twelve inches for the vast majority of us), a 3wt is great. I suspect most people would say that if you are throwing hoppers, and were chasing 20" trout, a 5wt would be more appropriate.

I am not most people.