Why R3 sits in the emitter leg of the transistor. Collector/emitter current flowing through it develops a voltage drop that raises the emitter above ground, which subtracts from the base-emitter forward bias. If the transistor starts to conduct harder (from heat or device variation), the larger drop across R3 pulls the bias back down, so the stage sets and stabilizes its own operating point. That degenerative action is what is meant by self bias, also called emitter bias.