You can do this by using variables, firstly place down a variable block and select 'shift all axis'. In the chest the a variable, I would name that 'pos1', and then a game value: 'target block loc', Then also put three numbers in the chest, the first being 1, second 0 and third also 1. Now place another variable and select 'shift all axis' again. In the chest put a variable, I would name it 'pos2' and then a game value 'target block location'. Then put three numbers in the chest again, the first being -1, second 0 and third also being -1. Now you can use the 'set region' game event with the two position variable to set an area to a certain block in a 3x3 grid.
If you have any questions, reply to this post or dm me.
