Jim,
see if you can lay hands on the volume
"Triplanes"
by Bowers and McDowell
It is NOT only about triplanes, but a fairly exhaustive study of 3 or
more winged aircraft. Quite a selection of photos, some plans.
Interesting and worth looking for.
BTW, if you find plans of the large Caproni Triplane seaplanes used by
the British post WW1 I'd love to hear about them !
Regards
Shane