Let $x$ represent the number of liters of $25\%$ saline solution that is added to the three liters of $10\%$ saline solution. Then the total volume of the $15\%$ saline solution will be $3 + x$.
The volume of saline in the $3$ liters of $10\%$ saline solution is $(0.1)(3~\text{L})$.
The volume of saline in the $x$ liters of $25\%$ saline solution is $(0.25)(x~\text{L})$.
The volume of saline in the $3 + x$ liters of $15\%$ saline solution that is obtained is $(0.15)[(3 + x)~\text{L}]$.
Since combining the $10\%$ saline solution with the $25\%$ saline solution yields the $15\%$ saline solution, the volume of saline in the $15\%$ solution must be the sum of the volumes of the saline in the $10\%$ solution and the $25\%$ solution, which yields the equation $$(0.1)(3~\text{L}) + (0.25)(x~\text{L}) = (0.15)[(3 + x)~\text{L}]$$ Can you take it from here?