can we check is blocks are placed in 3 x 3 x3 ? a recipe to make for example a big water tank..

if i want to make a big tank..

the size would be 3 x3 x3...

how can i  make a script that check if there is for example obsidian placed 3 x 3 x 3 of size ?

when they are placed i woul acces the water tank's gui.