I just downloaded Lightroom 5 Trial, I have lightroom 3 already on my Mac. I noticed my computer is low on space and wondered if its because I downloaded the trial. Lightroom 5 now has all my photos in it- are those copies of the photos in Lightroom 3? Did the download copy over all my catalogs?
No, that would not happen. Lightroom does not copy your photos into its catalogs. If you had one copy of the photo before you did this, you will still have one copy of the photo.
What Lightroom does is creates small thumbnail versions of your photos (called "previews"). Depending on how many photos you have, and options that you set in the program, these previews may or may not take up a lot of space.
Why don't you use your operating system to see what is taking up space? Report back, and we can advise you further.