We start with a set of three segments that will serve as the fixed lengths of our three bars. Construct three segments a, b, and c in the Canvas.
Now, we need to create three new segments that are attached to each other and have these three lengths. To do this, we note that if a segment of fixed length is attached to another of fixed length, then the attachment point lies on a circle. Thus, to create the first segment and an attachment point for the second segment, we create a point A, a circle with center A and radius the length of segment a, and then attach a point B onto this circle.
Now, at B we will want to again create a circle of radius the length of segment b for the next segment. However, we also want this next segment to be attached to a third segment having length equal to the length of c. To do this, let's create another point C and a circle centered at C of radius equal to the length of segment c.
In the final step of this construction, we will want to have a segment with endpoint B of length of segment b, and this segment attached to a segment with endpoint at C of length of segment c. An easy way to do this is to create a circle with center B and radius equal to the length of segment b and then find the intersection of this circle with the circle centered at point C. Once this intersection is determined we create the three bars.
Note that when we create the third bar we need to make a choice of which of the two circle-circle intersection points to use. If we now grab point B and move it we will see the linkage moving much as a physical linkage of bars would move.
However, when point B moves too far around circle A and the intersections disappear, then the segments based on these intersections will also disappear.