These three species are difficult to distinguish from one another without the use of a microscope. Under the microscope, Amanita bisporigera can be distinguished by its two-spored basidia; macroscopically it tends to be more slender and delicate than the other two species. Amanita virosa and A. verna are difficult to distinguish from one another, but a drop of KOH on the cap of A. virosa turns it bright yellow, as in the accompanying photograph. I have never seen A. verna, so I don't have a picture of it.