They all work for this speaker of AmE, although I prefer #2 the least. 'Should' seems redundant.
"I don't know" means I don't have that information.
"I wouldn't know" means I don't have that information, and there's no reason to expect why I should have it.
We usually say the latter when somebody asks us a question about something we're either not interested in, or we are upset that we don't know it, such as being excluded from an information source.