What gas was least abundant 3.5 billion years ago in the earths atmosphere?

2 million years is hardly anything when considering the age of the earth. If you are considering the early earth and its atmosphere you should speak of billions of years.

The inert gases (on earth) are the result of radioactive decay and build up in the atmosphere over time. Argon is (comparatively) rather abundant, Helium and Neon are fairly abundant and have found commercial applications. Xenon, Krypton, and Radon are rare but have been collected and have had an impact on human history. Health nuts use to breath pure Radon and suffer rather gruesome radiation poisoning.