I installed it the normal way, then moved the entire directory to dropbox (easiest if using Terminal on Mac)
cd ~/Libraries/
mv GoldenCheetah ~/Dropbox/
then symbolically link the directory to Dropbox:
ln -s ~/Dropbox/GoldenCheetah/ ~/Libraries/GoldenCheetah
Repeat on all computers, except you don't have to move anything - since Dropbox will sync up GoldenCheetah/ folder automatically. Just install, remove ~/Libraries/GoldenCheetah/ folder and create a symbolic link in its place.
3.0 works fine this way, but I do wonder what will happen if multiple instances of running GC will try to save data - there may be some changes that may be overridden if you don't remember to quit it in some location, and make some changes elsewhere, then quit the existing, older GC session.
Oleg.