Pick two sides. Add the both of them. If their sum is greater than the only side left, then it might be compatible. You have to repeat this step for the other remaining sides.
You said 3cm,10cm, and 15cm.
10+15 is equal to 25, which is greater than 3. So it's correct.
15+3 is equal to 18, which is greater than 10. Still correct.
However, the sum of 3 and 10, which is 12, is not greater than 15, which means that the lengths given may not form a triangle.
Hope this helps xx