Why should we use 3.3 V instead of 3 V?
Why do we use 3.3V in circuits? Why not 3?

Why not \$\pi\$ V? Why not 2.8 V, a much nicer number than 3 V? The more things like power consumption or speed matter, the less you align with &quot;human-pretty&quot; numbers, and more with physical needs.
In this case, the physical need is actually &quot;something slightly above 3V, but less than 5V, to save power in our new LVCMOS circuits&quot;, ca 1970.
Point is: when TTL (transistor-transistor logic) was still the dominant technology for integrated logic, supplies lower than ca 4.5 V were impossible, due to large collector-emitter voltages in the bijunction transistors used there. Hence, with a bit of headroom, 5.0V became a standard.
Now, CMOS was introduced, and it could work well with supply voltages down to ca 3 V. People wanted to give a little headroom.
So, my guess here is why it's 3.3V, and not 3.15V or 3.4 V: they picked a voltage for which there were already voltage regulators ready, in the drawers: 3.3V, which had, interestingly, already been (one) supply voltage of the Apollo Guidance Computer, so NASA and early semiconductor companies had poured in money into building these.

TL;DR: &gt; 3 V: Need that, 3V is just a tiny bit too low for reliable CMOS logic gates at the time of invention. 3.3 V: probably because hardware for that voltage already existed in the early 1970s.