Need help placing billboard gui between multiple parts?

Do simple math for example :-

if the part 1's position is = 1331, -379.5, 3398 and part 3's position is = 1331, -379.5, 3389 place the part 2 is the position = 1331, -379.5, 3389

Here my part's X,Y position is same i just changed Z position

Here the part 2's Z position is :-

{ (Part 1's Z position) - (Part 3's Z position) / 2 }

In case you are changing all of the positions then to set part 2 in middle set the part 2's position

X = [ (Part 3's X position ) + { (Part 1's X position) - (Part 3's X position) / 2 } ]

Y = [ (Part 3's Y position ) + { (Part 1's Y position) - (Part 3's Y position) / 2 } ]

Z = [ (Part 3's Z position ) + { (Part 1's Z position) - (Part 3's Z position) / 2 } ]

Only apply this math in the different position between Part 1 and Part 3