I bought a 3 day pass in Prague. On the back it says it's valid for zones P, 0, B. I have no idea where these zones are. On the internet, someone said it covers beyond the airport. But that doesn't mean transportation to the airport is covered. So is it? Can I get on the bus to the airport with this pass?
Generally, yes, you can take the bus from within Prague to the Václav Havel airport with the 3-day ticket. Though the special AE line is an exception, your ticket is not valid there.
DPP (the main transportation company in Prague) has a page about fares:
Zone P includes the metro, trams, Prague public transport buses (lines No‘s 100 – 299 and 901 – 915), the Petřín funicular, ferries and selected railway line sections.
Zone 0 includes buses from the periphery areas (lines No‘s 300 – 420 and 951 – 960) running through the Prague area, and selected railway line sections.
So, if you take a bus numbered from 100 to 299, you know for sure you are safe.
DPP's page about transport to the airport lists bus lines 100, 119 and 191.
For lines numbered from 300 to 420, according to this map, the airport (marked as "Letiště") lies within the "P, 0, B" zone, so you can use those lines too.
The one line you can't use is the special Airport Express (AE) line, which has special fare.