Whereabouts do you live? At a guess if Brechfa is on the way home then you'll be passing by Nant-Yr-Arian.
If you like your XC riding I'd say you should definitely make a call in at NYA, its one of my favourite XC trail centres. If you prefer jumpy style trails then Chli-Macx and Brechfa are more up your street.
With regards to CYB, you will struggle to get 3 days of riding there. If you do the Beast it takes in all of the bits of the other trails apart from the stuff on the Tarw. IMO the Tarw is only worth doing if you've got the time and energy as it does have a few sections of (nearly?) 'original' CYB trails (notably Snap, Crackle & Pop), however it does have an awful lot of fireroad ascent and descent. The last 2 descents are worth a look but can easily be done without the whole route.
Penmachno has some really good riding, make the effort to do the extended XC loop.
If I were you I'd make it a day at CYB doing the Beast and a bit of Tarw if you can be bothered, then a day taking in Penmachno and Marin. On your 3rd day do the big xc loop at NYA - it has some classic XC trail centre trails but also takes you out into the wilderness on some land rover tracks. However if you like your jumps then call in at Chli-Macx or Brechfa. Brechfa is longer but you can do laps of Chli-Macx via the short-cuts.