Hi Zach,
The circuit is very simple and since it operates at a Very High Frequency, depends more on its wiring layout than anything else, due to stray capacitance and inductance in the wiring.
People have reported building a quantity of circuits that are all identical, but some work well and some not.
Since the 3rd transistor operates at a low current and does not have a tuned load, it won't add any range to the transmission, but will isolate the antenna from the oscillator so that it doesn't change its frequency when you get near its antenna.
You can't just simply add a tuned load to the 3rd transistor. Then it will have an output exactly the same as the 2nd transistor. You need to change its bias to Class-C, and to operate it at a high peak current that will melt the little 2N3906. So it should be a power VHF transistor with a heatsink as well. Then the output power will be high but illegal.
Cx filters the supply voltage so that it doesn't swing all over the place at audio frequencies, especially when the battery is getting low and its internal resistance is higher. The 10nF capacitor that is in parallel with it does the same thing, but at VHF frequencies. Since the 10nF cap operates at VHF, it must be a ceramic type like most other capacitors must be.