Inspiring Questions from the Sea of Examinations——题海拾贝

Bob, Fred, and Vi play a game in which a card numbered 2,3,4,5,6,7,or 8 is stuck to each of their foreheads. Each player can see the other two players＇ numbers but not his or her own. An observ er tells the three players that the three numbers are not all different and that their product is a perfect square（安全平方数）, how many of them can deduce the number on their foreheads？