Can you take a picture and post it, please?
There could be many different possibilities. Fear, heat and stress can cause one to shed fur.
Do you groom him on a regular basis?
Chinchillas are capable of allowing their fur to "slip" (fall out) when they are being hunted, leaving the predator with only a mouth full of fur.
When they are not being handled improperly, the fur will often slip.
If they want down and you continue to hold on to him/her, or you may be holding him to tight, or he may be trying to escape, the fur can 'slip' during any of these instances.
If you pull on the fur, I would be surprised if it did not come out.
Chins going into and out of prime will shed fur, but that is all over and a little bit at a time, not all in one place.
It almost sounds like something/someone may have accidentally pulled a small amount out, leaving the surrounding fur trying to 'fill in', spreading out, until the new fur can grow in to replace the fur that was removed.
Is all the fur in that area the same length? Fur chewing is also a possibility.
How old is Benji ?
Jo Ann
It is imperative the you make sure all three are males ... if there is a female in the same cage with two males ... the two males may fight to the death over her, any time she goes into season. Unfortunately, she, if there is one, might get caught up in the fight also.