Partition into triples. Example:
ll = RandomInteger[{1, 10}, 30]
(* Out[1]= {3, 3, 8, 3, 5, 1, 1, 10, 9, 1, 8, 1, 7, 10, 2, 7, 4, 5, 6, \
4, 4, 1, 5, 1, 5, 9, 5, 4, 5, 5} *)
Get local maxima:
Flatten@
Position[Partition[ll, 3, 1], {a_, b_, c_} /; b > a && b > c] + 1
(* Out[4]= {3, 5, 8, 11, 14, 16, 19, 23, 26} *)