Thomson effect
If a metallic wire has a non-uniform temperature and a current is passed through it, heat may be absorbed or produced in the different section of the wire and called Thomson heat and the effect is called as Thomson effect.

Thomson heat charge passed × temperature difference

Seebeck effect:
If two junctions of thermocouple are placed at the different temperature, then an emf will be produced in thermo-couple which will be proportional to the temperature difference

Peltier effect: It is reverse of the Seebeck effect. According to this, when an electric current is passed through thermo-couple, heat is evolved at one junction and absorbed at the other end i.e one end become hot while other become cold.

The major difference between the Thomson effect and other two is that in Thomson effect we deal with only single metallic rod and not with thermocouple as in Peltier effect and Seebeck effect.