Use /spreadplayers:

/spreadplayers x z spreadDistance maxRange target

where

• x and z: the x- and z-coordinates of the centre
• spreadDistance: the minimum distance between the entities
• maxRange the maximum horizontal distance to spread the entities from the centre specified in x and z (must be at least spreadDistance + 1)
• taget: the entities to spread

For example, to “summon” zombies at least 1 block apart in a 7 × 7 square centred at your current position, you need to first summon the zombies:

summon zombie

and then teleport them using spreadplayers:

spreadplayers ~ ~ 1 3 @e[type=zombie,x=~,y=~,z=~,dx=1,dy=1,dz=1]

Use /spreadplayers:

/spreadplayers x z spreadDistance maxRange target

where

• x and z: the x- and z-coordinates of the centre
• spreadDistance: the minimum distance between the entities
• maxRange the maximum horizontal distance to spread the entities from the centre specified in x and z (must be at least spreadDistance + 1)
• taget: the entities to spread

For example, to “summon” zombies at least 1 block apart in a 7 × 7 square centred at your current position, you need to first summon the zombies:

summon zombie

and then teleport them using spreadplayers:

spreadplayers ~ ~ 1 3 @e[type=zombie,x=~,y=~,z=~,dx=1,dy=1,dz=1]