Use the array to help you work this out. 3 lots of 7 (or 7 lots of 3) is 21. There are 3 missing values. So 3 x ? = 18. Use the inverse operation to work out the missing value. 18 ÷ 3 ...

We end up with them around a metre from the rear wall and well away from the sides. They work just fine in our 3 x 7 x 5m (hwd) test room, but only really gel when we are seated more than 2.5m away.