Since no one has yet answered, here is some details.
According to Shelby Stanton's ORDER OF BATTLE U S ARMY WORLD WAR II (1984),
"3rd Coast Artillery Regiment (Harbor Defense) (Type B)
Stationed at Fort MacArthur Calif under Harbor Defenses of Los Angeles and upgraded to Type A in 1941; regiment, less 1st and 3rd Bns, redesignated there as as 521st CA Battalion 18 Oct 1944; 1st and 3rd Bns redesignated 520th and 522nd CA Battalions respectively."
In other words, this regiment was a regular army coast defense unit stationed in the Los Angeles area at the start of the war, manning fixed coastal defense guns and some antiaircraft batteries. It remained there until it was broken up into separate battalions in 1944. These battalions did not go overseas and 521st and 522nd were disbanded 15 September 1945, and the 520th CA Battalion was again redesignated 3rd Coast Artillery Battalion on 1 December 1944 and also was disbanded on 15 September 1945.
There are others who frequent this board who are much more knowledgeable than I about such things, especially US Coast Artillery matters, so hopefully they will speak up.