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Here some links I found for earlier versions that supposedly run on osx 10.3.9 :
Free: http://mac.wareseeker.com/free-google-sketchup-for-mac-os-x-6.0.277/&ei=FdGqSYS2OYKEsAPps9XgDw&sig2=1D2kcG_MaLQM3Kl3Mt76zw&usg=AFQjCNFk6ZwmWnoIMN7dYCH9KaLtyranzA
Pro:
http://en.kioskea.net/s/google-sketchup-older-version-for-os-10-3-9&ei=FdGqSYS2OYKEsAPps9XgDw&sig2=Yy6fa8RciGMsyJf7Kmv6rQ&usg=AFQjCNFj_1SdkDBp9CMjHkWQCaMMvA36BA