When talking about 3-point amplitudes (e.g. three gluons) I`ve come across the fact, that if we have three lightlike 4-momenta $p_1,p_2$ and $p_3$ they should satisfy
$$p_1^{\mu} + p_2^{\mu} + p_3^{\mu} = 0,\tag{1}$$
which looks like one of the momenta has a negative energy.
This makes me wonder, because I thought, that
$$p_1^{\mu} + p_2^{\mu} = p_3^{\mu} \tag{2}$$
makes more sence. I can´t figure it out on my own, so help would be appreciated.