Dunjinni forums have tons of symbols - most are photorealistic so they don't mesh super well with the profantasy symbols, but they are also free.
In general - All of the ad-ons for CC3 give you extra symbols and tools to make mapping easier. When your first starting out with the program, they can be a big help (i.e. using the wall and floors tool rather than manually drawing a wall or floor with the line tool and polygon tool respectively) - but you don't actually "need" them to make dungeons. Having said that, I do highly recommend them and the annuals.
My best advice is to watch and follow along with all of the youtube tutorials for CC3 (particularly the Dungeon one). Once you understand what he's doing, you'll be able to make your own and the program gets easier to understand with every map.