using variables you should use the $()-Operator, i.e. Min( $(Variable1), $(Variable2), ....). Second, and even more important is, that the Min()-Function will only work inside a chart-formula. Because it is an aggregate-function. Third, proper working depends of the content, if you have only plain figures (i.e. constant values) in your variables, it won't work as well. Try two test-cases: in a text object and in a formula of a chart box.