The "throw" of road and mtb triple FDs are quite similar, but the little arm that the parallelogram sits on will be further out on the mtb FD, meaning that the parallelogram of the mtb FD will not swing as far inboard, and the parallelogram of the road FD will not swing as far outboard. If you have a mtn crank and a road FD, i wouldn't be surprised if the thing won't swing out far enough. Fixes? Get an appropriate mtb FD; one with the pull and swing you need, that'll handle a 48t large chainring. Or, if you have a square taper crank, you may be able to use a BB with a shorter spindle, depending on the requirements of your crank. Grabbing a dual-pull, high-clamp XT with the 48t large-ring capacity would probably solve your problem most easily, but it's hard to say with the available information.