You can set Q(x, x, 2x).
QA=( 1-x,2-x,3-2x),QB=(2-x, 1-x,2-2x)
QA * QB =( 1-x)(2-x)+(2-x)( 1-x)+(3-2x)(2-2x)
=6x? - 16x+ 10
When x=4/3, there is a minimum value.
So the coordinate of Q is Q(4/3, 4/3, 8/3).
Have fun! I hope I can help you. If you don't understand, please ask. I wish you progress in your study! O(∩_∩)O