What do these components of an AB amplifier do?
This is a simplified Class-AB push-pull amplifier circuit.

What is the purpose of R2.

R2 is there to bias Q1. Without it, the Q1's base will not see GND and thus cannot get into DC operating point. Also R3 brings a low-cut in conjunction with C1.

Likewise, what does R6 and C3 do? It seems to be part of the feedback circuit?

Yes. The feedback network consists of R5, R6 and C3. At DC C3 is open-circuit. So R6's effect gets cancelled out. This makes the output to be fed directly to the inverting input (Q2's base) at DC, which means that the gain at DC is unity.
At AC, C3 brings a low-cut in conjunction with R3. The cut-off frequency is \$\mathrm{1/(2\pi R6\ C3)=1.5Hz}\$ with a slope of 3dB/decade. Actually we can assume that C3 is a short-circuit along the full audio spectrum, because it's high enough and it does not impact any single point inside the audio spectrum. So at AC, the output is fed to the inverting input with a division ratio of R6/(R5+R6), which means that the voltage gain at AC is (1+R5/R6) - just like in an opamp non-inv amplifier.

Is Q3 working as a constant current source?

No. It's called &quot;Voltage Amplification Stage (VAS)&quot;.

What does C2 do to help?

It's called Miller Capacitance. Its main purpose is to ensure stability. Think of it as a capacitor across the B-E of Q3 but with a capacitance of \$\mathrm{C2m=C2 × h_{FE-Q3}}\$ (Miller Effect). This C2m brings a high cut. But C2 is way too high in the circuit shown, so probably it'll chop off all the usable band. Normally a few tens to a few hundreds of pF is enough in practice.