$ perl -E '
$_ = q{1 AC2 34 TRP A 6 ALA A 7 ILE A 14 GLY A 15};
say for m{(?: (?<=\A) | (?<=\s) )( (?:\S+\s+){2} \S+ ) }gx;'
1 AC2 34
TRP A 6
ALA A 7
ILE A 14
GLY A 15
$