You might even want to rip up the 3" boards and replace them with plywood that is glued and screwed to the bottom layer of wider planking. My concern is that planking on top of planking might be flexing and/or swelling etc. and that 1/4" backer board on top of it will only partially mediate this underlying situation. You could also use this as an opportunity to better address your finish floor height. Maybe use 1/2" plywood and 1/2" backer instead of your current 3/4" planks and 1/4" backer. This would definitely be better. If you don't replace the 3" planks then ring nail or screw the bejesus out of the floor to make sure the 3" planks are tight down and not flexing. Also make sure to glue down the backerboard with the recommended adhesive, I think they recommend latex modified thinset mortar, plus nails or screws. My advise may seem like a lot of work, but imagine how much work it will be to do it wrong, rip it up, re-do your subfloor, and then redo your tile. Do it right, do it once.