To find the total weight of all the ants in the world, we can multiply the average weight of an ant by the total number of ants.
The average weight of an ant is \(3 \times 10^{-6}\) kg, and the total number of ants is \(1 \times 10^{16}\).
So, the total weight \(W\) can be calculated as:
\[ W = \text{(weight of one ant)} \times \text{(number of ants)} \]
\[ W = (3 \times 10^{-6} , \text{kg}) \times (1 \times 10^{16}) \]
When multiplying these numbers in scientific notation, you multiply the coefficients (3) and add the exponents (-6 and 16):
\[ W = 3 \times 1 \times 10^{-6 + 16} = 3 \times 10^{10} , \text{kg} \]
Thus, the approximate weight of all the ants in the world is:
\[ \boxed{3 \times 10^{10}} \text{ kg} \]