You have a long focal length range covered with the zooms you currently have.
If I were you, I would enjoy two of the best L zooms ever made and buy a 16-35 L (my favorite lens ever) to complete your kit. However, if you dead set on buying primes, do this....Buy a roll of high quality gaffer tape
. Why high quality gaffer tape? Because it won't leave a bunch of nasty adhesive goo all over your equipment. You'll use it for all kinds of things in photography but for now, you can use it for two things...
1. Use a short length to "lock" the zoom at the focal length of different prime lenses you are interested in. Shoot all day like that. See if you can accept that focal length before you buy/rent one. Then rent one. Then buy one if you really need it.
2. Cut 1/4" wide strips that will fit around the shiny black/red ring circumference on your black L lenses and protect them when they rest/touch on a table, etc and hide the red ring from potential thieves
. (Also, put a small piece over the 5D
and Mark III
logos on the body too!)
If you want to experiment with a prime without breaking the bank, the EF 28mm f/1.8 USM
is still one of my favorite lenses and it is only around $300 used. My suggestions for great L primes would be a 24 f/1.4, a 35 f/1.4 or a 15mm f/2.8 FishEye. I've never warmed up to longer primes because I do a lot of casual indoor shooting of people and I don't like to stand 15-20 feet away from them. Once you get past 24-35mm, you will start feeling rather boxed in with what you can shoot candid unless you are doing studio work. You'll back up into people, walls, small dogs and children and eventually off of curbs and cliffs trying to get everyone in the shot or compose it a certain way. People will wonder why you ask them for a photo only to run away from them and shout from across the room to get the shot. Really. Unless your goal is to fill the frame with one face, not a couple or a threesome.
Take a look at what Pete Souza has done as the White House photographer with a 5D Mark II and the 35 f/1.4. He shoots with that more than anything. (You can look at the EXIF data to see how he shoots.) He also uses the 50 f/1.2 but he's shooting in larger venues than most. http://www.flickr.com/photos/whitehouse
In conclusion, try the gaffer tape on your zooms, simulate the focal lengths and then see what works best for your style. Don't buy three $1000+ primes just to try until you are sure that you need/want them. You may find that primes are too much trouble and the 16-35 is a much better idea!