Some people collect them in panels and some collect them as singles. You can find graded examples of both on ebay. Another Topps issue that came in 3 card perforated strips was the 67 Punch Out set. Singles can be graded in that set too, but not the 3 card strips, presumably because it would require a huge slab.
Other examples would be the 61 and 62 Topps Stamp inserts that came in two panel perforated form but can be graded as separated singles as well
