Dead Hedgehogs

I've recently seen dogs "playing" with a hedgehog; not nice. 

Could be a fox has had a go at them,they could have been shocked to death by a fox attack?

Surely if there are no marks on two of the bodies it looks more like poison of some description, could be from anywhere, they do roam. The gnawing may have been after it was dead.

Badgers and foxes turn them over and eat them  soft belly first which doesn't seem to be the case.
I would take them to your vet , he may be able to tell  from just looking what happened to them.  Vets are usually good with helping with wildlife and will do it free of charge.

I think you need to find out if it was poison but dont know what you can do if it was

foxes , dogs, badgers ,slug pellets , other garden poison

apart from the obvious they could have been just too young , 

Eating slugs poisoned by pellets?

It can happen.

The recent documentary on TV showed vulcher numbers in India were in decline. The end reason was a drug the animal hurders used to kill them off gently made its way into the meat and organs of the dead animal and post mortem showed it was responsible for causing gout in the the vulchers' kidneys and slowly they all but disappeared leading to a rise in leapard attacks on children and dogs and rise in breeding dogs. doing so they have foolishly messed up their entire ecosystem through the over use of pesticides, insecticides and other drugs. now they are trying to re-introduce them. sad really - lack of education for the unwise who use this stuff and profits first for those who sell this stuff.

Might be a similar situation here regarding pellets.

regards to killing slugs i use salt on them if i see them or throw them in the green wheelie bin. never used slug pellets near edible food.