For that you write the values for the parameter in the HTML as
onclick="LaunchModalDialog(firstParam, secondParam, thirdParam)"
This way, when it would trigger, it would pass on the values.
It can be anything, integer, string etc. What so ever you want to send to the function.
Also, please note that the values for attributes must be qouted
<input id="Button1"
onclick="LaunchModalDialog(firstParam, secondParam, thirdParam)"
value="button" type="button" />
You have only 3 parameters, that's why I have included only 3, you can add more or less depending on the nature and type of function you're using.