Sankyo is one of those core Aikido techniques that can give newcomers (or even people who've been practising for a few years) real problems. The main problem is not the sankyo grip itself but how to retain control throughout the technique. Often people 'take off' the sankyo as they pull down the arm, thereby making the technique ineffective. I encourage my students to practise the sankyo control as two separate movements: 1) rotate the arm around the pivot point established by the sankyo grip close to the wrist. Only then 2) point the arm towards the ground to move the uke.
Experienced practitioners take the uke down in a single movement - that is, they both rotate around the pivot-point and simultaneously take the arm down towards the floor. But in my experience beginners find this too complicated to master as a single movement, which is why I encourage them to learn sankyo in the two steps outlined above.
Today I came across an alternative way of explaining the sankyo control. In this video (one of an excellent series made by the New York Aikikai), the instructor comments on the similarity between the sankyo 'cut' and a sword cut. "I don't want to point the fingers down and cut with the butt of the sword," he says, "I want to try to cut out with the tip and the elbow of the sword."
If you are having problems with sankyo, this video is worth watching...