The "/roll 1d3" has to be at the beginning of any macro created and not anywhere else. The [[1d3]] is the inline version of the roll command and can be placed anywhere in a macro so you could write a simple macro "/me attacks with acid and does [[1d3]] splash damage". If you are looking for more information on macro creating look in our wiki here and if you need more specialized rolls, you can find Dice reference material on the wiki also.