Envelope #1 I reckon.......
If the check was in envelope 1, that would make statement 1 false, statement 2 false and statement 3 is the only true statement.
If the check was in envelope 2, statements 1 and 2 would both be true.
If the check was in envelope 3, statements 1 and 3 would both be true
That made my head hurt........ I remember that one from school and it took me a pen and paper to remember it all !