Third House synastry governs social ethics, or your ideas about how people should behave toward you and you toward them. The partner’s planets falling into your Third House will show the types of manifestations to which you instinctively react as if he weren’t your partner but simply a familiar or distant relative, so to speak, “a person in general,”.
In close relations (such as within a family) this can be completely inadequate. On the other hand, in conflicts, when a partner seems to be an enemy, activating his planets in your Third House can immediately soften the situation because you will see that the enemy has a human form, sensing a commonality between the partner, yourself and the rest of humanity. Naturally, this is an excellent start for peace negotiations.
Practical training is also governed by the Third House, meaning that through the synastric planets of your 3rd House the partner could teach you a lot, as long as you are open to it. You may also have an impulse to teach your partners something; but their perception of these attempts depends primarily on how their planets which fall into your Third House stand in their own natal chart.
If your partner’s Seventh, Eighth or Tenth Houses correspond to your Third House, your attempts at “practical training” can be mistaken for something different. Here the subtlety lies in the fact that you can always learn, perceiving any situation as an educational one; but teaching your 3rd House synastry partner requires their consent, and they may not be inclined to give it at all.