T3 Sonar is used on Naval maps to provide intelligence on Naval threats. The Seraphim are the only faction without the T3 sonar. All three factions T3 Sonar has a different side ability and the ability to move slowly.
All three T3 Sonar platforms have a decent view range with a large sonar range. This sonar range is its primary asset and allows one to spot naval units that are advancing under and on top of the water. T3 sonar has the largest sonar range (obviously) of all sonar stations. Also they all have a slow movement speed that allows them to relocate to where they are needed. This is useful at times, do not expect it to allow you to move far though as it is quite slow.
after you have a T2 sonar there is almost no excuse not to upgrade it to T3. it uses the SAME amount of power as a T2 one and as the uprade only cost 400 mass the only reason you should not is the T2 sonar you have covers all the water on the map.
Other than the typical variations in health cost and time, all three factions sonar have a different special ability. The UEF version has a Angler Torpedo launcher with a DPS of 38. The Cybran version features a stealth field that is larger than other stealth fields in use. The Aeon version is more protective as it comes with anti-torpedo charges. The Seraphim lack a T3 sonar platform.