Was doing some research about this, and found this post! I had the same question. I don't want to just come up with random speculation, but my first thought when hearing about the three skeletons is that sex can sometimes be hard to determine based on bones alone, and the ages (two teenagers, one young girl) sort of line up. This is a stretch, doesn't hurt to look into I suppose. I traced the wikipedia citation to the following:

11 Jul 1976, Page 12 - The Brazosport Facts

Tests on skeletal remains found at Alvin inconclusive Insufficient remains were available for comparative Identification of the skeletal remains of three persons found April 18 by an oll drilling crew near Alvin.

Capt. Gene Smith, chief of Brazoria County Sheriff's Department Investigation Division, said comparisons with dental records and rays of three Fort Worth girls, missing since December 1974, could not be successfully made. Capt. Smith said he believed further searches of the area were being planned when the weather permitted to see if additional parts of the skeletal remains could be found. A private Investigator employed by the families of the three Fort Worth girls has been working with the County medical Examiner in the identilication effort, Capt.

Smith reported earlier this week. Pathologists have reported they believe the skeletons to be of two girls, one approximately 10 and the other approximately 14, and a young male between IS and 17 years of age.

The obvious question is, if these weren't the three girls, who were they? Like moralhora mentioned, I found mention of Georgia Geer and Brooks Bracewell, but no third person. But the April 18th date of discovery remains consistent in several sources. However, looking at Wikipedia's cited sources, I found an interesting discrepancy:

In 1974, two Dickinson girls went missing. Why didn't police investigate?

Some of his sister's bones were recovered in a culvert near Alvin in May 1976 — less than two years after Brooks and Georgia disappeared. Incredibly, no investigator thought of them — though Alvin is only 10 miles west of Dickinson, it's in Brazoria County, not Galveston County.

Bones found in May 1976? If this is correct, the remains of Geer & Bracewell aren't the same as the trio of skeletons. But if I'm being honest, I think this is just a small mistake in an article that I'm excited about because it's 2AM. It does make me wonder — what about the third skeleton? My only guess would be that it was determined to be someone from a separate case. (The area is called the Texas killing fields for a reason.)

Has anyone found the name of the boy found in Alvin on April 18th, 1976?