I'm a bit sad nobody thought to use MapIndexed[]:
MapIndexed[With[{k = #2[[1]]}, If[Mod[k, 3] == 1, "string" <> ToString[k], #]] &,
Range[9]]
{"string1", 2, 3, "string4", 5, 6, "string7", 8, 9}
J. M.'s missing motivation ♦
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