The following horses won the Kentucky Derby and the Preakness but did not win the Belmont:
1932 Burgoo King Did not start (unknown reason)
1936 Bold Venture Did not start (injury)
1944 Pensive 2nd to Bounding Home
1958 Tim Tam 2nd to Cavan
1961 Carry Back 7th to Sherluck
1964 Northern Dancer 3rd to Quadrangle
1966 Kauai King 4th to Amberoid
1968 Forward Pass 2nd to Stage Door Johnny
1969 Majestic Prince 2nd to Arts and Letters
1971 Canonero II 4th to Pass Catcher
1979 Spectacular Bid 3rd to Coastal
1981 Pleasant Colony 3rd to Summing
1987 Alysheba 4th to Bet Twice
1989 Sunday Silence 2nd to Easy Goer
1997 Silver Charm 2nd to Touch Gold
1998 Real Quiet 2nd to Victory Gallop
1999 Charismatic 3rd to Lemon Drop Kid
2002 War Emblem 8th to Sarava
2003 Funny Cide 3rd to Empire Maker
2004 Smarty Jones 2nd to Birdstone
2008 Big Brown DNF to Da’ Tara
2012 I’ll Have Another Did not start (injury)
2014 California Chrome 4th to Tonalist
http://en.wikipedia.org/wiki/Triple_Crown_of_Thoroughbred_Racing#Near_Misses
Poor Barbaro would have done it.