Well, the answer to this problem depends on many things, none of which you mentioned
- CPU on your computer
- amount of computer memory
- type of original file, RAW, TIF, PSD or JPG
- size of original photos (in pixels, width and height, not in megabytes)
- size of exports (in pixels, width and height, not in megabytes)
- disk space that is free
It has nothing to do with LR3 being on the same computer.
Normally, Lr5 does take a little longer to export PV2010 photos than Lr3 did (e.g. 10-15% longer), and it takes substantially longer to export PV2012 photos (e.g. 30-40% longer) - rough estimates..
Dunno if that's what you are experiencing, or whether you are having abnormal Lr5 performance.
As Paige said - nothing to do with Lr3..
As well as what Rob and DJ have said, there's also a trick for speedier exports. Instead of exporting 700 photos, export in 3 sets, 200, 200 and 300 or some variation on that. Total export time is reduced, but computer is pegged. Lightroom doesn't apply full processing to export because it leaves CPU so you can still work during Export. This tricks works if you don't want to keep working.
Sean, if I may be so bold, I think you left a word out of your description
I think it should say: "Instead of exporting 700 photos, simultaneously export in 3 sets, 200, 200 and 300"