Enhanced Vegetation Index (EVI) is an algebraic equation of:
EVI = G X ((NIR - RED)/ (NIR + C1 x RED-C2 x BLUE + L))
Because it is an algebraic equation you should only get one value.
Where NIR is the value of the near infrared band (Band 4), red is the value of the RED is the value of the band (Band 3), BLUE is the value of the blue band (Band 1), (all three bands are atmospherically-corrected), L is the canopy background adjustment. Additionally G is the gain factor, C1 and C2 are the coefficients of the aerosol resistance term. For more information go to the Wikipedia page here or here for a thread on how to implement the equation manually.