Limit data validation values to a 1 point margin in 3 different cells

I have a section of my program that asks for board scores (point value is 6-10 in increments of .5) I have set up a data validation so that users can only enter those specific values. Problem is, none of the values can be more than 1 point from each other. (the part I have yet to figure out)

Example, if the first value is a 6.5 the second is 7.0 and the third is a 9.0, I need the data validation to display an error because the 9.0 is beyond the one point spread.

Summary: Display an error message if any of the points are more than 1.0 point from another.

Thanks, you guys have always been great at hepling me!