Old crochet terminology: "3 under 3 chain"
After trying to work it our for a while, I gave up and redid the pattern.

With the new configuration, I've come to the conclusion it means "3 treble on the space left between the first 2 stitches (which are a 3-chain), then chain 6". This and the following 2 instructions yield this result, which looks like the top of the leaves pictured in the guide: