This form of jealousy goes beyond unhealthy and starts getting into the realm of psychological illness. Someone suffering from such extreme jealousy isn't in control of themself, is a slave to their own misery and can't see things rationally.
With this degree of jealousy, the slightest thing - an unknown number, for example - can be proof he's having an affair. If a waitress dares to smile at him in a restaurant you naturally assume she's sleeping with him. You twist reality to such an extent that you can't see straight and you lose all sense of reason. This is third-degree jealousy, and it's downright dangerous.
Third-degree jealousy goes hand in hand with hate and revenge, and in some cases can lead you to act on your feelings (just look at how many crimes are motivated by jealousy). Before you get to this stage, you need to seek help!