I have an iPad 3 with iOS 8.3; I wonder if there is some software/tool that allows to install Adblock and how to enable it on Safari (if I can of course).
There are apps which aren't content blockers, but which rather install a VPN on your device and route your traffic through this to block ads. This is still supported on devices which don't support content blockers.
For other devices which do support content blockers on iOS 9, there are many options available.
To add a content blocker, install it from the App Store, then choose Settings → Safari → Content Blockers and enable the content blocker. For apps which do not use content blockers, see the app itself for installation instructions.