3' is pretty insane for cel size... the largest 
I've shipped was probabbly 2' long x 10" wide.  I 
taped 2 priority mail boxes together and stuck a 
solid piece of cardboard inside, behind the 
junction so that the package could not be bent.  
Cel arrived to the buyer fine.  Theoretically you 
could do this with 3 priority mail boxes, but 
only if the cel isn't very wide.