6 cm, 8 cm, 12 cm
The perimeter of the reference triangle is ...
... (15 +20 +30) cm = 65 cm
Then the similar triangle has a scale factor of ...
... (26 cm)/(65 cm) = 2/5
Multiplying the side lengths of the reference triangle by 2/5, we get ...
... {15 cm, 20 cm, 30 cm) × 2/5 = {6 cm, 8 cm, 12 cm}
These are the side lengths of the smaller similar triangle. (Check: their sum is 26 cm.)