That will be treasury 2 that is attacked, and the third (3) statement is correct. If the robbers attack treasury 1 the first and second statement is correct while the third is incorrect. If the robbers attack treasury 2 the third statement will be correct, while the first and second statement will be incorrect. If the robbers attack treasury 3 the second and third statement will be true while the first will be incorrect.
This leaves the third statement as the only possibly true statement.