Combining three fields , g,E and B is really only E and B. The g field can hardly influenced the effect due to E and B. The acceleration due to g is only 9.81ms-2 as compared to acceleration due to E and B which is easily around 1 x 1014 ms-2. To sum up the effect of these three field is mainly to sum up the acceleration due to E and B bearing in mind their directions. Therefore need to do a vector summation using pythagoras theorem and tangent of angle.
