You can try a few things:
1) find the distance between 2 coordinates in either 3D or 2D space (search 'distance between 2 points' or 'distance formula' or something of the sort on the forum)
2) Try something like this. X#, Y# and Z# are the positions of the object that needs to enter the trigger area. Say the trigger area is at coordinates 50,120,80 and you want to check and see if you enter an
imaginary box around that point that's 20x20x20 (you don't actually make a box with this method...)
if X#>=50-10 and X#<=50+10 and Y#>=120-10 and Y#<=120+10 and Z#>=80-10 and Z#<=80+10
DO ACTIONS
Endif
Why + and - 10 all the time? Because 10 is 1/2 of 20.