Let us construct the required triangle as follows:

Step 1:
Draw the base line XY of the length 3 cm.

Step 2:
Use your compass to make an arc with centre at point Y and the radius 4 cm.

Step 3:
Use your compass to make a circle with centre at point X and the radius 5 cm.

Step 4:
The point of intersection between the arcs is named as X.
The triangle is completed by joining all these three points.
Now, from above construction we see that the triangle formed is a right angle triangle.