actually this is possible.
in one setup we use 9 Projectors connected via 3 Tripleheads to one computer. each projector gets a softedged and 4point corrected image.
for one triplehead, you have to make your comp the right size (e.g. 2400x600) and then define one input slice per projector and the overlap. note that with the overlap, you will lose some resolution in the end, depending on how wide the overlap is.
on the output settings in arena, you then have to make one screen and assign it to the triplehead.
in this screen you then have the three slices that you need to move to the projectors and adjust the edge blending.
best, bildwerk