Another way just using a global match with an alternation of look-behinds rather than split.

$ 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
$
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I hope this is useful.