If you're getting 9 sts per inch and want a finished measurement for a 3 mo size (you really should make a larger one, infants don't fit into 3mo sizes for more than a month) then you probably want to make the biggest size at a minimum. You didn't say what size needles get you 9 sts per inch, but maybe you ought to try 4 or 5s. BTW, the pattern gauge is 18 sts over 4" so your 9 per inch is 36 sts over 4". Unless you mean something else.
CO 24 sts and measure how many sts in 3 or 4" to be sure.