So if I get three monitors that have display ports, I won't need an active adapter then?
Yes that’s correct.
You would connect 2 via Display Port and 1 via DVI
however as azeem40 said you only need 1 monitor to be Display port, then you don’t need any adapters.
If you’re buying it all new they you will want 3 of the exact same display's for a symmetrical look (just means 1 will use DVI not DSP).