can we sketch a triangle with 1:2:3 side-length ratio????

Intuitively, no. Think about the angles of the triangle.

If the sum of two sides is equal to the length of the last side, then the "triangle" is a straight line.

But the question is, when i draw it with compass, it becomes a triangle- -!!!

I don't think you are doing it right. ONe side 3, the sum of the other two is 3 also. No way a triangle.

A necklace is 60 centimeters long and the beads on the necklace are 50 millimeters apart. How many beads are on the necklace?