Hey its easy identify all the possibilties fo all the coins.
YUU WILL GET
HHH HHT THH HTT TTT HTH THT TTH
WHERE H= HEADS N T= TAILS...
you can see there's only 1 possibility out of 8 that the first two will show tails n d last one a head n the probablity that only 1 coin show tails is 3/8