this is just my hypothesis, but i think dduks and plave are trying to build PLAVE Virtual Universe (for easiness, i'll short it to PVU), just like how M*rv*l have their Cinematic Universe. 

In PVU phase 1, there will be 3 trilogies, the first one was Asterum trilogy. next is Caligo. and there'd be another trilogy as the end of PVU phase 1. so it'd be 3x3 = 9 arcs/albums.

the reason why i thought there'll be 3 trilogies with 9 arcs? 
1) 3v3r's name not just meant to be the mirror of rêve, but it also signifies the 3 x 3 structure in the plave universe
2) in yejun's notes he was highlighting 1+3+4+1 = 9
3) since plave lore revolves around cube, it'll be make perfect sense to have 9 arcs to complete the "story" cube 

in short:
3v3r
1 + 3 + 4 + 1 = 9
3 × 3=9

3 trilogies
9 albums