Option 1: Taste and odor control. Chloramines, which are formed by combining ammonia and chlorine, do not play a significant role in taste and odor control in water treatment plants. Other methods, such as activated carbon filtration, are typically used for this purpose.

Option 2: Weed control in reservoirs. Chloramines are not used for weed control in reservoirs. Herbicides and other specific treatments are employed for controlling the growth of unwanted vegetation.

Option 3: Disinfection. Chloramines are indeed used in water treatment plants for disinfection. They are effective in killing bacteria, viruses, and other microorganisms that may be present in the water, making it safe for consumption.

Option 4: Removal of permanent hardness. Chloramines are not used for removing permanent hardness from water. Methods such as ion exchange or lime softening are commonly employed to address water hardness issues.

Overall, option 3 is the correct answer as chloramines are used in water treatment plants for disinfection purposes.