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Mapping the Trajectory
The trajectory of each link was mapped as joint was mapped for every step made by the crab from the video. An example image with the joints on each leg marked is shown below.
The joints are connected by links. At the time of mapping the trajectory, we did not assign any type for these joints. To make it easier for us to map the trajectory, we developed a MS -EXCEL VBA program to enable us to easily mark these joints and extract (x,y) coordinates of these points. A total of 5 points were extracted. The MS EXCEL VBA program and the values can be found in the attachment below. [Book1.xlsm]
Synthesis
The trajectory mapped 5 points, which leads to 5 different link positions. In order to apply the Graphical three-position synthesis, we are considering the first , third and the final positions as shown in the figure below for the rear leg. 
It should be pointed out that the graphical three position synthesis is usually applicable only for one link in a mechanism consisting of revolute joints. But since our requirement is to have one mechanism replicating the motion of three links, we decided to independently carry out three-position synthesis for these links and merge them on to the same mechanism. The links are assigned measurements 2.5in, 4in and 3in for the top, middle and bottom links respectively. The three position synthesis for the rear leg is shown in the figure below. The corresponding AutoCad file is also attached.