?, PSV, EHK, TWZ, ILO

GIven series is an overlap of 2 different series following same pattern. 
?, EHK, ILO & PSV, TWZ

Each cluster is of 3 alphabets which have 2 alphabets skipped amongst each other 
And every consecutive cluster starts with the alphabet which is chronologically after the alphabet in the center of previous cluster
For eg., the cluster starts with I which is chronologically after H - alphabet at the center of previous cluster. 
EHK starts with E, hence, the alphabet at the center of cluster in place of ? should be D
Hence, the cluster there must be A-D-G