The difference between v2 and v3 is that v3 will no longer allow uBlock to modify network calls. Previously one of the big savings of a adblocker is they can stop the calls from being made AT ALL. That means less tracking AND less of your internet bandwidth being used

Both v2 and v3 can block the actual elements from your screen so you will probably still not see most of the ads. But you will still be able to be tracked and your data will still be used in the background