How long will this pump last if used every day? I have had this pump for three years and I am now having a problem. It does not shut off after starting to pump. I have to unplug it to stop it. What does this sound like? A plumber told me these pumps don't last for much more than three years if used every day?? Is this true?

Check the float, it may be going bad.

I agree with truck. You can get new float swithches. I think I have even seen them at Home Depot. Put the float where it needs to be on the pipe, plug it in the electric outlet and then plug your pump into that. Of course, remove your current float first. Piece of cake!
To answer your question, there is no definitive time limit on pumps. The more they are used the shorter their life will be. I have changed pumps less than 2 years old and I have changed pumps more than 20 years old. One thing is certain, they all go out. And usually at the worst possible time.
When you install your new float switch make sure to locate it where sewage will not fall on the float, collect there and not allow it to rise. Also make sure it has free movement. No rubbing on the pipe or the side of the basket. Good luck!

And I agree with both Chayn and Truck. Here's a owners manule;
http://www.femyers.com/pdf/pdf.sse/sse.manual/25581a04.pdf
Contact Myers and order out a #218138139 float switch.
Good luck, Tom