I have my laptop - perfect color, my right monitor - a dell - perfect color and then my left - another dell - colors are way off and dull. If I drag photoshop to the other two, colors are perfect. Monitor colors match on all 3 monitors/screens. In Photoshop workspace is the difference. Placing, opening - colors all match, but the workspace in left Dell is dull. Dragging Photoshop to the others, beautiful once more, and output from all three is perfect. It's only the workspace.
Sounds like a bad/broken profile is being used for the third monitor...
Susan, you have a great question. If you look in the Related questions (you can find it in the column to the right, just scroll down a little), you will see many people have asked questions along these lines.
The answer lies in the software profile your computer uses to get your monitor to display colors correctly. These profiles are a complex subject, but luckily there are fairly easy ways to get this perfected and consistent.
You need to research monitor calibration tools. These take the guess work out of monitor brightness and contrast settings. They do practically all the work for you!
I also have multiple monitors at home. Here is what I settled on as a solution (for me, at least).
I have designated my biggest and nicest monitor as the location where I edit all of my photos. I use a calibration wand to periodically evaluate this display and the make adjustments so it will always show true color.
My calibration tool of choice is the Pantone Huey. It is super simple to use and not too expensive.
I hope this helps! (and if you print photos at home, you may need to look into getting the right color profile for your printer -- but that is a slightly different subject!)