Explanation:
Yes it is possible to have a triangle with 3 cm, 6 cm and 7 cm sides because in order to form a triangle the length of two sides of a triangle should exceed the third side as shown below :
3 + 6 > 7
9 > 7( True) ✔
6 + 7 > 3
13 > 3 ( True) ✔
7 + 3 > 6
10 > 6 ( True) ✔
Hope it will help :)