What version are you using? With ver 11.2a it works for me. The result needs a little tidying, but basically the text is all there.
Jon
Program:
text:stack("text1","text2","text3")
WRITEPRN("c:\temp.prn"):text
Resulting text file:
// Datafile written by Mathcad
// 05/25/05 12:05:42...