I read this expression in a text. It seems to be an old proverb about monkeys. But I am not sure about its meaning. Why 筋, for instance? Does it means muscle? reasoning faculty? Why would they be lacking?
It means: "Monkeys are less clever than humans" or literally: "Monkeys have 3 hair less than people"
足らぬ being used for 足りない
and 筋 being the counter for hair
To add a bit of history/legend:
毛 doesn't stand for "hair" but for apparel, appearance or skill...etc.
The legend states that during the creation of the world, Kamis (god(s)) thought human and monkeys were too similar and removed 3 details (毛) from them.
A monkey asked to "god" (kami) why they could not become human. God answered, because you lack the following 3 points (毛):
(reason, compassion and sense of accomplishment)