I was between these three cars when last week (longer, as I research a car heavily before purchasing)
I read the Car & Driver compario between these 3 exact cars, and after a test drive of them all. I bought a Fiesta ST.
Ford had the edge as Last month I was invited to a FORD event and was able to take the Fiesta and Focus ST on a parking lot Track. Speeds where 0-55 through the course. The Fiesta handled like no other car.
Quality, as in reliability? FORD all the way.
Quality as in interior bits and pieces, mini, then Ford, last Fiat.
Overall ride quality, Mini if you like soft and comfy, Ford if you like a firmer ride that extrudes please bash me into corners. FIAT if you don't like driving for very long.
About the only thing the Fiat 500 has over the rest is style. It looks like no other car on the road. But I guess you can say that with a Mini too.
If you like performance, do not look at the Fiat 500. The Ford will walk all over the mini in every category that requires turning the wheel.
So with all that said. I chose a Fiesta ST due to the reported reliability, compared to the issues mini and fiat people are having.
Plus, optioned to how I like a car, the Mini was 3 grand more than the Fiesta.