Three of each application within right click 'Open With' menu

Try resetting LaunchServices: The X Lab has a FAQ on resetting it for previous os's.

Mac OS X 10.6 Snow Leopard

I don't think anything has changed in Lion or Mountain Lion from the Snow Leopard instructions wrt restting the launch services. I ran the command and didn't suffer ill effects.

Things like OnyX and Tinker Tool may have a GUI button to press if you don't want to copy and paste into the Terminal.

It works for me too, but I get the duplicates back after a day or two. Any idea what's causing the problem in the first place?

MST

Do you have multiple copies of applications floating around, like in the Downloads folder?

Ah, I see. I do have other copies on partitions of my hard drive where I keep old versions of the system. When I reboot the computer these partitions are all mounted automatically and I guess this is where the duplicates are coming from.

It seems a bit odd; surely there's a way either to stop LaunchServices looking anywhere except the boot disk, or (more reasonably) to stop unwanted disks and partitions mounting when I reboot the computer.

MST

Barney, I have the same problem and I tried the solution you offered as well as the one right below it. I am running 10.8.2 and neither solution gave me any resolution at all. I'm really stuck on this because the menu with 3 each offerings is so long that I have to scroll down through it now. Help! ~pam

I've got nothing more than that.

When you reset LauncServices using the command posted above, did you get any feedback on the command line?

You should have seen nothing for a while, then the prompt reappear when it completed.

I notice in the command above, there are two / in the path. I don't think that would cause a problem, but here it is without the extra /

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain user

Barney, that did the trick, it must have been the extra slash that was not getting the job done. It took 25 seconds and cleaned out everything that was tripled up! I wish people who put solutions in text for us to copy and paste would be more careful about the exact characters they are typing. Thanks again... If I could give you an extra 10 points, I would! BTW, my whole system seems to be running zippier, too!!