You're on the right track... purpleheart, osage, ipe, bloodwood, cherry, bamboo... all make good core lam woods, and I've used 'em all.
I tend to stick with the denser woods for heavy warbow weight tri-lams. My fav combos are: maple(hickory)/purpleheart/osage, maple(hickory)/bloodwood/osage, hickory/osage/ipe, hickory/bamboo/osage. These combos have worked for me up to120#.
If you only want 50#, I wouldn't do a tri-lam anyway IMHO... just back some osage with maple or hickory, and you're good to go.