Engineering your own voynich

[nablator]

Am I on the right track at all with this? 
1) Somehow divide the plaintext in groups of three characters
2) Use three conversion tables, one for each position

It's possible to do this unambiguously in several ways.  I'm not sure if it is possible with 26 characters or less for all 3 positions. Maybe ' ',  '.' and ',' are included in the character set of the plaintext?