Class 3 defensive was a classification of armaments used on some Federation starships in the 24th century.
In the mid-2360s, the USS Lantree, a class 6 Federation supply ship (a variant of the Miranda-class) was one Federation starship armed with class 3 defensive armaments. When the Lantree sent out a distress call in 2365, it was inferred that these armaments would not be very effective in combat. (TNG: "Unnatural Selection")
Presumably this armament classification included at least the six dual phaser emitters on the saucer and the two aft emitters below the impulse engine of the ship that were visible on the model.