I have Adobe Photoshop CS3 and wonder if it is worth getting Adobe Lightroom as well.?

Lightroom is intended for use with a DSLR camera. If you have such a camera, I highly recommend getting LR.

It can natively edit the raw format files from many camera brands. It can also access the metadata from image files and lets you add tags. The library mode lets you examine and compare various images, and apply mass edits. The edit mode lets you make the sort of adjustments needed by photographers. The output modules let you set up print orders or export for use on the Internet. All of this happens without making changes to the original files—edits are kept separate, preventing you from accidentally ruining your work.

Lightroom doesn't have the ability to make graphics from scratch as Photoshop does.

Photoshop does everything Lightroom does, so no.