return - Line breaks in text expansion -

i'm noob having tough time expaning multiline paragraph when use `n comand isn't recognizing in msgbox works fine script:

::a:: msgbox, line1 `nline2 `nline 3 

produces message box saying : line 1 line 2 line 3

but

::a::line 1 `nline 2 `nline 3 

produces: line1line2line3

what doing wrong?? thanks