1. Get CDock for free from Source Forget (click the green download button)
2. Unzip the app you’ll need to right-click and choose to “Open” to circumvent Gatekeeper restrictions
3. Click the “Dock theme:” submenu and choose “Yosemite 3D”, then click on “Apply”
4. The Dock will refresh and magically turn 3D again
You can also choose a transparent dock.