As mentioned, unless the arrows are specifically labelled 3L-18, I'd recommend purchasing 3-18's as well. The "L" would be the weight code, with all "L"s grouped by weight to fit the ACC weight tolerances. Just like the DQ and DR are slightly different batches by weight as someone else also mentioned.
If you plan on mixing old and new batches of arrows it would be best to try to stick to the same weight code, although 1 letter off may be close enough to not be noticeable if you don't have a choice. I've mixed batches before on some 2-18 Hyperspeeds that were 1 letter off and didn't notice a difference until I got out to longer distances (i.e. 80yd for NFAA field).
fyi, the 3L-18 I believe does use less carbon, or a lighter carbon, and is therefore lighter and slightly weaker in spine compared to a 3-18. 3 refers to the number of carbon wraps around the shaft I believe, so both use 3 wraps, just different weight.
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