Next is tpg.
The first letter of each is 3 more than in the previous one; the second letter is 5 more than previous second letter; the last letter is 1 more than the previous.
a b c d e f g h i j k l m n o p q r s t u v w x y z
a b c d e f g h i j k l m n o p q r s t u v w x y z
a b c d e f g h i j k l m n o p q r s t u v w x y z