3×1 Weave
What does 3×1 Weave mean?
Referring to the number of weft threads per warp thread, in which three threads cross every one (hence, 3×1 weave)
Heddels explains 3×1 Weave
Denims that have heavier weights (over 10.5 Oz / square yard) are 3×1 weaves, especially compared to denim with a 2×1 weave (under 10.5 Oz / square yard).
Additional Resources
Source: commons.wikimedia.org