A size 12 foot by 5-foot rectangle ad takes 12*5 = <<12*5=60>>60 square feet since the area of a rectangle is l*w.
If each company bought 10 ad spaces, the total area that their ads took is 60*10 = <<60*10=600>>600 square feet.
If each square foot of ad space was charged at $60, 600 square feet of ads cost $60*600 = $<<60*600=36000>>36000
If all three companies each purchased an equal number of ad space, together they paid $36000*3 = $<<36000*3=108000>>108000
#### 108000