What is the role of IC3 in the attached circuit ?

Can any body explain the role of IC3 (Nand Gates) in the walkie talkie circuit attached below.

I would be appreciated for any help.

walkie talkie circuit

N1=Oscillator (10.7MHz)
N2=modulator(10.7MHz and output from voice controlled oscillator)
N3,4=paralleled antenna driver.
Cheers,
Richard

Thank you so much dear Richard.
As a last question can you please introduce me some alternatives for the transistors utilized in this circuit. They are so seldom in my city.
Best regards.

The transistors used (audio and wideband)are very common, if they are not available locally, you should be able to get them via mail order via the web.
Otherwise you have to look at the data sheets for each device, and compare them against the data sheets for parts that you can get locally. This is not for the faint of heart. There are some tables of equivalent parts available in various books, but they are not necessarily very reliable, so you still have to use the data sheets in the end,
cheers,
Richard

Thank you so much.
Best wishes.