I believe that SSViewer (the class that interprets your templates) doesn't handle more than 2 parameters.
Basically, SSViewer parses the template with Regular Expressions. At the current time, these are limited to expressions with 2 parameters or less.
You could probably modify SSViewer.php to handle more parameters if you wanted, but it is unusual to need them. The expressions are kept deliberately simple so it is easy for non-coders (designers, etc) to use. If you start building complicated functions with lots of parameters, you start to loose the compartmentalization.