How To Calculate kVA values in power analyzer meters?
The original question has been edited to more clearly indicate the \$\sqrt3\$ is the prefix and not plain ordinary 3. However, the principle for calculating VA based on using RMS values (rather than average values) remains the same.

3*V*I (or \$\sqrt3\$VI) is the correct formula for calculating VA if the voltage and current are RMS quantities but it seems you are using average values and these are likely to be average values of RMS values and so will give an error.

Consider 10 amps flowing for 1 second then zero amps for 9 seconds repeating. The average is clearly 1 amp. The RMS is somewhat different because you have to square then average then take the square root.

So 10 amps squared is 100 and this is averaged over ten seconds to be 10 amps squared. Take the square root and you find that the RMS is \$\sqrt{10}\$ amps and is substantially different to the average value of 1 amp.