If you still have your serial number you can download the trial version of Lightroom 3 and activate it with the serial number that you have. If you want Lightroom 5 you would have to pay the price for the upgrade which is typically in the $79 range. But you would still have to know your Lightroom 3 serial number in order to activate an upgrade version.
You will find downloads for all the LR versions on the Adobe Updates page. To get back to where you were, download the last version of LR3 to avoid installing the earliest and then having to update to the latest: