Nama (mentality; citta and cetasika)(名法) and rupa (materiality including the 4 main elements that make up the material words)(色法) are dependent on conditions (因緣所生法), hence impermanent/sunyata (無常/空).
The statement which describes/represents these truths (三法印) is made upon a recognized concept (概念法); it is meant for inference (推論) and epistemologically cannot be grouped into any nama or rupa category (非名色法義), therefore, there is no need to say whether a statement of truth is permanent or impermanent. This is the same as to question whether 1 + 1 = 2 is permanent or not.