where can i download aperture 3.0?

Aperture needs to be purchased; there have never been downloads for the installer, only for updates.

It is much to late now. Apple stopped selling Aperture more than two years ago, when the development of Aperture has been stopped. You can still buy the early versions of Aperture 3 on installer disks from eBay or Amazon, but on High Sierra you would need Aperture 3.6, and the update to Aperture 3.6 has never been released outside the AppStore, so you could not update the CD versions purchased from eBay bond Aperture 3.4.3, and they are incompatible with High Sierra.

Nowhere, unless you previously bought it using the App Store app, and now it would be the last version, 3.6, from the Purchases section.