It's really kind of impossible to compare the three:
An American crooner/actor who sang country, gospel and pop from '53-'77 vs. a British band who innovated the development of modern rock and psychedelia from '60-'70 vs. an R&B/pop singer who dominated the 1980s video era.
Not to mention, they all had vastly different cultural impacts:
Elvis served as the commercial breakthrough for rock and roll music and helped break the segregated lines between "white" and "black" music, The Beatles revolutionized the music industry (moving away from soloists and professional songwriters to groups who wrote their own songs) and laid the foundations for many modern styles of rock music, and Michael Jackson made similar strides in popularizing and evolving pop and R&B during the video era.
Really, a question like this will always come down to personal preference.
For me, I guess I'd choose The Beatles for sheer consistency. Elvis and MJ had numerous ups and downs in their career and sometimes produced less-than-stellar material, but The Beatles had a solid and steady (if brief) output which ultimately influenced most of the genres I love, including progressive rock and power-pop. I definitely don't agree with the consensus that they're the "greatest band of all time", but it's hard to dispute their influence on popular music and popular culture, which I think was a much bigger impact than either Elvis or MJ.
