Only 30" 16:10 center fits "perfectly" with 20" 4:3 side monitors in portrait-landscape-portrait, 27" 16:9 supposedly looks somewhat ok with 19" 4:3 sides but it's far from tidy.
Also, PLP means no native Surround or Eyefinity support, so you're at the mercy of SoftTH or stretched windowed modes. That's the reason why I ended up ditching my 20-30-20 PLP setup, it's just too much of a headache compared to matching screens with modern games.