how is this a string raise???

[SNOWBALL]

Someone limps to me from the SB in a 3 chip 6 chip structured 15/30 game. I am in the BB and I throw out one chip, and another and another to raise. Villain calls string.

IMO this should NOT be considered a string, because I was unambiguously raising. However, villain said that because I put the chips in one at a time and didn't announce raise, that my raise wasn't valid.

Your thoughts? I have never seen this come up before.