Hey there! Great question!

First off, make sure you are not using a floating tremolo bridge (like a Floyd Rose whammy system), as since the bridge is floating, it will be very difficult to bend more than one note in tune.

Hand strength is a big factor in being able to pull off these multi-string bends smoothly and cleanly. One exercise would be to just repeat the triad bend over and over, moving it up and down the neck (between 7th and 15th frets - lower frets would be very difficult because of the higher tension), focussing on trying to make the bends sound as even and in-tune as you can.

Drilling this simple exercise a few minutes a day for a week, or even a month, can make a dramatic difference in hand strength and bend control.

Let me know how it goes!
Mike