I understood that the current CX-70 front derailleur may have enough travel though it is listed for a double. Earlier versions of 2X Ultegra front derailleurs worked. I am using an FD-6500 and Ultegra 3X9 shifters (ST-6510) to shift a Tiagra triple crankset though the chainline on the Tiagra is only 145mm where that of a modern MTB crankset is normally 150mm.

This is part of the problem with modern cranksets with integrated spindles; you are stuck with the manufacturers chainline. With older 3 piece cranksets you could get different length bottom brackets to adjust the chainline to fit your bike and needs.