To find the perimeter of a triangle, you add the lengths of all its sides together. For the right-angled triangle with sides of 3 cm, 4 cm, and 5 cm, the perimeter can be calculated as follows:
\[ \text{Perimeter} = 3 , \text{cm} + 4 , \text{cm} + 5 , \text{cm} \]
Calculating the sum:
\[ \text{Perimeter} = 3 + 4 + 5 = 12 , \text{cm} \]
So, the perimeter of the triangle is 12 cm.