I like how neat and organized your have your query and that you included they are African American, all clues good when you're working with a common surname like Green.
But, you could include a little more. TX is a big state, 254 counties, so if you could include the counties, like you did for Harris and Galvewton, that would also be good. I looked and there is a Cold Springs in Coryell Co, and a Sheppard in San Jacinto. Are those the right locations?
Are the three Greens related? I ask to see if we can narrow down time period for them. Oh, and when you say 70s or early 80s, is that in the 1900s or 1800s???? One looks for different records in those time periods.
There are several good researchers that browse this forum and have access to more sources than I have. With a little additional information, maybe one of them could help you.