From Förberedande kurs i matematik 1

When we have three fractions involved in an addition, we need to multiply the top and bottom of each fraction by the product of the other fraction's denominators so that all fractions have a common denominator,

\displaystyle \frac{1\centerdot 4\centerdot 5}{3\centerdot 4\centerdot 5}+\frac{1\centerdot 3\centerdot 5}{4\centerdot 3\centerdot 5}+\frac{1\centerdot 3\centerdot 4}{5\centerdot 3\centerdot 4}=\frac{20}{60}+\frac{15}{60}+\frac{12}{60}

When all three fractions have a common denominator, they can easily be added up

\displaystyle \frac{20}{60}+\frac{15}{60}+\frac{12}{60}=\frac{20+15+12}{60}=\frac{47}{60}