Exactly what Type 3 diabetes means depends on who you ask.
The most widely used definition of Type 3 diabetes
Overwhelmingly, those in the diabetes community use Type 3 to describe any family member or close friend whose life has been affected by living around people with diabetes. This is the definition we are using. If your life has been changed by having to deal with the ins and outs of the disease experienced by someone you’re close to, then you’re Type 3.
Type 3 diabetes… or Type 1.5?
Some in the diabetes patient community have also used it to describe someone with Type 1 who experiences the insulin resistance and other symptoms associated with Type 2. More often, however, that type of diabetes is called “Type 1.5“, because it’s “halfway between Type 1 and Type 2”.
Yet another definition of Type 3
Some in the scientific and medical community use “Type 3 diabetes” to refer to the possibility that sometimes Alzheimer’s disease may be linked to insulin resistance in the brain. While this term has been used since 2005, very few in the diabetes patient community use the term in that way. Plus, this entire theory is still in the very early stages.
See the Wikipedia entry for more information.