  We measure triangular flow relative to the reaction plane at 3 GeV
center-of-mass energy in Au+Au collisions at the BNL Relativistic Heavy Ion
Collider. A significant $v_3$ signal for protons is observed, which increases
for higher rapidity, higher transverse momentum, and more peripheral
collisions. The triangular flow is essentially rapidity-odd with a slope at
mid-rapidity, $dv_3/dy|_{(y=0)}$, opposite in sign compared to the slope for
directed flow. No significant $v_3$ signal is observed for charged pions and
kaons. Comparisons with models suggest that a mean field potential is required
to describe these results, and that the triangular shape of the participant
nucleons is the result of stopping and nuclear geometry.
