What operating system are you using?
32-bit or 64-bit?
How much RAM do you have?
Have you purchased a new computer?
Here are the system requirements for running Lightroom 5:
Since you already have Lightroom, you can purchase an upgrade version for $79, available here:
If you are installing on a new computer that hasn't had Lightroom installed on it, you will need serial numbers for both your Lightroom 3 as well as the serial number for Lightroom 5.
I've been waiting for my husband to help me figure out the answers to your questions. Thanks for letting me know the system requirements. Do I have to buy all the versions in between Lr 3 thru 5? Or just go from 3 to 5?
Each version of Lightroom, including dot releases, is a completely independent program that doesn't rely on previous versions. You can purchase Lightroom 5 and install it. Purchasing an upgrade license will require accessibility to the Lightroom 3 serial number, which will be readily available if you are installing on the same computer. The installer will find it automatically.