doesn't seem that difficult.

$ perl -le 'use strict;use warnings;my($s,@r,@t) = "1 AC2 34 TRP A 6 A
+LA A 7 ILE A 14 GLY A 15";@t=split/\s+/,$s;while((@r=splice @t,0,3)==
+3){print join(" ",@r);}warn"leftovers: @r\n"if @r;'
1 AC2 34
TRP A 6
ALA A 7
ILE A 14
GLY A 15
[download]

what's the catch?