Hasn't this been done before? Oh well, whatever. If this is CW Grievous he wins since he beat Ki-Adi-Mundi, Shaak Ti, K'Kruhk, Tier Seir and Aayla Secura all at once, and those five Jedi > AOTC Obi-Wan and Anakin.
On the other hand, if this is ROTS Grievous he could lose. ROTS Obi-Wan beat him with some trouble, I'm not sure if this duo is stronger than ROTS Obi-Wan. But I'll still go with Grievous. ROTS Obi-Wan was a Soresu Master and still had trouble, Anakin doesn't know Soresu and in AOTC Obi-Wan isn't very good with it yet. Anakin'd probably charge at Grievous and get pwnd, stabbed and killed. Then Obi-Wan would fight Grievous and last a short while cause he uses Soresu but would definately lose.