Can I set up one Quickbook username to manage three organizations?
With a single QB (Intuit) username you can add as many company files and in any flavor as your wallet can stand.
Additionally, if you add yourself as an accountant user (with a different email address) you can get a free QBOA accountant special version to more easily manage all of your company files. (plus you can get discounts)
Yes you may use the same email address as Master Admin to manage those 3 QBO accounts. If you have to open a new QBO account, make sure to click the Buy button to get the discounted price for up to 3 months. If you purchase from the Trial account, you have to pay the regular rate.