I could be very wrong here, but the only 3" DA machine I came up with in a quick Amazon search was the "Zota" for $79.99.
A Griot's 3" lists on their site, and here on AutoGeek for $139.99.
That is a difference of only $60 for machine that has a lifetime warranty, and some of the best customer service in any industry. For a tool that you plan to have for several years, a mere 60 bucks should not be the deciding factor between a unit that already has you searching for a better attachment (BP) right off the bat.
If an adaptor even could be found, it would probably be in the $20 range. Couple that with aggravation of it all, and the (now after the BP adaptor) $40 savings seems none too good. Right?
I learned many, many years ago: Never go (dirt) cheap on power tools or lawn equipment.
You do not need to buy the FLEX 3" machine, but you'll be in misery with the Amazon model................ JMO