You're right, there are three definition/letter mixes in each set of clues. They appear in order of the clue numbers, e.g. in the first triplet, the definition/letter mix for 1 across is at the start of the clue, the DLM for 6 across in the middle and the DLM for 10 at the end.
I also did about a quarter and put it aside, and I'm not sure I'll pick it up again unless I get bored watching my grass grow. I love Azed's puzzles and have solved his puzzles every week for many years, but I can't stand DLM puzzles, even when the surfaces are as clever as this. I wonder how many people will solve this without resorting to electronic anagram crackers.
If you persevere, good luck!