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According the the Haynes manual I have the 3V knock sensors are located in the same place under the intake manifold as the 2V but it's just has one one each side vs just on one side in the 2V.
I confirm what Monster-4 said. Under the intake manifold, one on each side of the "valley" of the cylinder block. There is a boss on either side, with the bolts threaded straight down.The 3V bosses are a little further back, like almost halfway down the length of the block, whereas the single on the 2V is closer to the front. The 3V bosses are slightly higher as well - almost up to the head gasket level.
As an aside, the boss for the 2V single sensor is still cast and tapped in the flat bottom of the 3V block valley, just not used.
If you take the alternator off, you can get a better view into the cave under the intake and see them back there. Mini flashlight and mirror will help!