3. Two investors jointly invest in a business. Then one of them adds 1 million rubles, which increases his share in the business by 0.04. When he adds another 1 million rubles, his share increases by 0.02. Question: How much more money must he add to increase his share by another 0.04?
3. Suppose two investors have a total investment of $y$ million rubles. Among them, the one who increases the investment initially has $x$ million rubles, and the total increase is $z$ million rubles.
Thus, the amount to be increased is $z-2$.

From the given conditions, we have
$$
\begin{array}{l}
\left\{\begin{array}{l}
0.04=\frac{x+1}{y+1}-\frac{x}{y}=\frac{y-x}{y(y+1)}, \\
0.06=\frac{x+2}{y+2}-\frac{x}{y}=\frac{2(y-x)}{y(y+2)}, \\
0.1=\frac{x+z}{y+z}-\frac{x}{y}=\frac{z(y-x)}{y(y+z)}
\end{array}\right. \\
\Rightarrow\left\{\begin{array}{l}
\frac{0.04}{0.06}=\frac{y+2}{2(y+1)}, \\
\frac{0.04}{0.1}=\frac{y+z}{z(y+1)}
\end{array}\right. \\
\Rightarrow\left\{\begin{array} { l } 
{ 4 y + 4 = 3 y + 6 , } \\
{ 2 z ( y + 1 ) = 5 y + 5 z }
\end{array} \Rightarrow \left\{\begin{array}{l}
y=2, \\
z=10 .
\end{array}\right.\right.
\end{array}
$$