PHDs in the US do not necessarily require professional experience. In most universities PHDs and Master's students share the first 2 years of studies.
Some ppl go to a PHD/MS straight from undergrad and some ppl work (usually a couple of yrs, so I'd say that you would be fine with 3 yrs). I went straight to MS in mngt from undergrad without any work experience, and I know that others in my class did as well. So it really depends. However, I can also tell you that usually admin criteria for PHDs are a bit more strict than for MS (or at least that's what it was like at Stanford). That is usually becuase PHD admissions are usually tightly correlated to funding and grants, whereas MS are not.
So in the end, I'd say concentrate on your academic interests, form a good case for a research focus, ace your GREs, leverage your professional experience, submit a clean and well revised essay, try to visit the school that most interests you if you can, and you'll have it made! (...I know...easier said than done!...)
Best of Luck!!