Being a beginner Sensei showed me an easy way to to do Sankyo so that the form comes out almost on its own. That of course will change when I am able to do it better hee hee.
We started from a katate dori and, skipping every step coz I am sure you all know them, I end up shoulder against uke's shoulder with his inside elbow on top of my shoulder and his palm on top of my chest where I apply Sankyo. I must admit that this way it comes clear to beginners the "theory" of sankyo.
This way tho you have to keep the arm close to you, but by doing it this way I learnt that the closer the arm is to you the less chance of moving Uke has. Moreover it is also easier to move along with Uke in case he moves...