Try this
select * from table t where ? IN (t.date, t.date2, t.date3)
Another way of solving is just delete the two variables created by SSRS and update your query to
select * from table t where t.date = @param1 or t.date2 = @param1 or t.date3 = @param1
Another method write your query
select * from table t where t.date = ? or t.date2 = ? or t.date3 = ?
Let it create the 3 variables. Then go to Dataset, under dataset properties goto parameters section. There update the parameter value of 2 & 3 to Parameter1. Next go in the parameters section and delete the automatically generated Parameter2 & Parameter3.