Phillip,

There were two 3rd Texas Brigades. One was part of Walker's Texas Division that fought in the Trans-Missisippi West ( Red River Campaign , etc) . However, I think the 3rd Texas Brigade you are looking for was the one that was part of the Texas State Troops . The 3rd Texas Brigade of Texas State Troops was formed in the Military District made up of the Counties of Angelina, San Augustine , and Nacogdoches. These troops never left the state as their job was just as you have discussed, to guard against invasion. The 3rd Texas Brigade was made up of three battalions that were named after their respective counties, e.g. the San Augustine Battalion. My guess is that is who camped out in the San Augustine area in 1862. These Texas State Troop outfits had a hard time keeping a full comlement of fit men because the men would join a regular CSA outfit and go off to one of the main theaters of combat. I'd imagine that by late 1862 or early 1863 (certainly after the Battles at Sharpsburg and Shiloh )that these State Troop units were manned by the very young ( less than 17/18 years old) those too old for regular military service( 45 + years old ) and the lame . As the horrific losses in the eastern theater began to mount ( especially for "Hood's Texas Brigade) the recruiting officers were encouraged to back fill the decimated ranks of the ANV where possible. Also , by late 1862, things were heating up in the "west" ( Tenn/Miss) as well , so, most able-bodied men were shipped of to these "mainline units" back east. The main exception being those troops( primarily cavalry units ) who fought in Indian Territory and those Texas Rangers/Troops who tried to protect the western frontier from the Indians and bandits.
Back to your 3rd Texas Brigade; my guess is that the independent battalion of the 3rd was the San Augustine Battalion , which would have most likely been made up of local boys and men.
I was not able to find any activities for the 3rd Texas Brigade; however, why don't you email Ron Brothers about your interest. He has put together a nice bit of information on some units in the 9th Texas Brigade ( Texas State troops) . Try this email for him ; rbrothers@1starnet.com
Email me if you hit a deadend. As old a city as San Augustine is , I'd expect that they'd have had a newspaper that might have talked about the troops who were camped near the city; especially if those troops were made up of local boys and men. Sorry for rambling . Obviously, I'm guessing at some of my observation . Try Ron Brothers, he seems to be one guy that knows something about these Texas State Troop outfits. Good hunting
Craig