java script function

Question

hi,
i am taking 3 textboxes, first textbox is multiline in this Textbox user write message for sending mobiles,
and second textbox is display characters length in first Textbox ,
i take maxium characters for one message is 150. and if user write write morethan 150 characters that time message send 2 times.
and Third Textbox default text is "1". 
 my aim is if user enter morethan 150 chacters, means he enter 151 character this time textbox3 display "2" upto 300 characters it
dispaly 2, and again user enter text more than 300 characters, this time also Textchange is 301th character. similarly
i give 3rd textbox name is messagecount.
please tell me how to write this code in javascript. please give a logic / code.
Thanks.