  The sequence of the zeroth Landau levels (LLs) between filling factors
$\nu$=-6 to 6 in ABA-stacked trilayer graphene (TLG) is unknown because it
depends sensitively on the non-uniform charge distribution on the three layers
of ABA-stacked TLG. Using the sensitivity of quantum Hall data on the electric
field and magnetic field, in an ultraclean ABA-stacked TLG sample, we
quantitatively estimate the non-uniformity of the electric field and determine
the sequence of the zeroth LLs. We also observe anticrossings between some LLs
differing by 3 in LL index, which result from the breaking of the continuous
rotational to \textit{C}$_3$ symmetry by the trigonal warping.
