Filters, use a conditional expression for the value using the IN operator

If the data type for your filter field is not string, then convert it to a string first, and then the value expression should work using Split:

So, in the filter for the Tablix, the expression will be:

CStr(Fields!MyFilterField.Value)

Operator: In

and the Value will be:

=IIF(Parameters!ShowAll.Value = True, Split("1,2,3",","), Split("1,2",","))