split serial.readString() output
This is a C question, not an arduino question, but anyway:

From the man page:

DESCRIPTION
   The  strtok()  function  breaks a string into a sequence of zero or more nonempty tokens.  On the first call to strtok() the
   string to be parsed should be specified in str.  In each subsequent call that should parse the  same  string,  str  must  be
   NULL.


You are only calling strtok() once, so it only parses the first value. You need to keep calling it until there is nothing left to parse.