i dont think that the answer given by b555 is correct,
bcoz. see as an as we are moving close towards the point charge q the electric field (due to q is rising greatly, and that that due to -3q is decreasing{ but in smaller amount} )
thus the field at the position of q will be very large, or mathematically not defined, this formula E= F/ q is used when we are finding the field due to a charged body at some distance from it,
also in the expression for calculating the field, the magnitude of the test charge is taken to to be negligible and the electric field ( for a charged body) is defined as E = lim ( q---> 0) F / q