output resistance of discrete amplifier
Update: if you are still having issues, remember that the DC design must be proper to get linear AC gain. The intermediate stages need proper impedance optimization for gain and DC bias and that has not been Done here.  Emitter DC drops are too high and input impedance too low so extra emitter R’s are needed and lower existing values.

Emitter followers can only source current while Re sinks current. (Like LDO’s)

\$Zout = \dfrac{R_{C2}} {h_{FE3}}||R_{E3}\$
For AC coupled output to a load , looking back into  Zout, a negative voltage now sources current from the external cap swing sinks current and if this exceeds I(Re3)
for
and = Re for -ve voltage.

Ignoring base emitter resistance etc.

Note this design is poor due to lack of active push-pull outputs. e.g. Op Amps have these. THis makes the negative supply rather useless.