I recently bumped up to Lightroom 5 and want to keep and use my Lightroom v3 for some stuff.
As everyone else has said, that's a silly thing to try.
Can I use the same catalog???
No, LR needs an updated catalogue that is not backwards compatible.
Obviously, Im not expecting the settings to transfer over..Or, do they in an abbreviated state?
If you load a LR3 catalogue into 5 it will convert it into it's own new format, but it keeps all your old settings and process version. In effect you'll see no change to anything in the develop module (excepting the newer features in other areas, eg book & map module etc).
If you want to use the newer develop features that arrived in Process 2012, you just click the small lightning icon in the bottom right of the histogram panel or select process 2012 in the camera calibration panel. LR5 then changes the options available in the panel and applies a 'best guess' conversion of settings you can refine to take advantage of the newer process version.
It really is a no brainer with nothing to loose assuming you can run LR5 fast enough. LR5 is slower than 3, but if you've tried it you'll know what to expect.
Back everything up and move forward.