thank you for your answer. It is great when there are really only three files, but actually I have hundreds of them, three was just an example. I have a solution now:
When reading in the files, use the wildcard in the LOAD statement
Table: LOAD ID, Value FROM ..\List*.txt (txt, codepage is 1252, embedded labels, delimiter is ';', no quotes);
All my lists are then sequentially read and their rows added to the table.
After that you can create another table for aggregation:
MinMaxValue: LOAD min(Value) as MinValue, max(Value) as MaxValue Resident Table group by ID;
The group by statement is important. Finally you have a table with all aggregations grouped by the ID.