I ran an old Shimano XT FD-M735 on a Santana tandem fitted with 52/44/28 tooth chainrings and bar end shifters (i.e., the front shifter is friction). That front derailleur was designed for 3x7 MTB, max 48 tooth large ring. However, it is unusual in that it could handle a 26 tooth difference between the small and large chainring; most triple front derailleurs can only handle 20 tooth difference. What I found was setup was critical. Usually the outer cage is set parallel with the large chainring. But you will need to twist yours to minimize chain rub; try turning the back of the cage further inwards. Of course, with indexed MTB shifters, you have limited trim so you may always have chain rub in some combinations. In which case you are stuck and may just not be able to use all the combinations. Seems like you would want to avoid the large chainring small cog anyway. Good luck.