You can actually add a currency but it requires a script:
In order for this to work you must prioritise the type of the summing you want to perform, using the '(' and ')' symbols.
When you use the "$" + Value1 the expression is evaluated to a string and afterwards this expression is concatenated as a string to Value2 and so on with Value3.
A simple solution would be to use the following expression:
"$" + ( Value1 + Value2 + Value3)
The first addition is a simple string concatenation while what is between the '(' and ')' symbols is a mathematical one.
Does this help?