feeding a pretty much all dry diet is probably the main reason your bearded dragon died. Though you had fresh water available, bearded dragon's don't always drink freely from standing water. Pellets and the freeze dried insects have almost no water, maybe 10-12% at the most. However, I think this is a problem that has been on going for quite some and he probably would have died if you were with him for those three days anyway.
A necropsy will deffinitely indicate exactly what caused the death, providing you haven't frozen or buried your dragon by now. Keep it refrigerated and get it to a vet for a necropsy asap, the longer you wait, the more deteriated the body will be and thus, make it harder or impossible to determine cause of death.
ALso those canned insects have alot of preservatives which in the long run, may be harmful to bearded dragons. I can't say for sure, as to my knowledge no real steadies have been done on the long term effects of consuming that stuff will have on dragons.
It is always best to feed a variety of both insects and greens to bearded dragons, the more variety the better. Proper supplimentation, lighting and heating are also very important. Insects are have about 60-70% water, greens as much as 80% water and this is where dragons get the majority of their water from. Being arid to semi arid living creatures, standing water is not something they encounter often in the wild, morning dew is pretty much the only water they may encounter for most of the year.
-----
PHLdyPayne