San Antonio is the Texas city that was Spanish and Mexican long before it was American. The Spanish founded a mission and a fort here in 1718, and in 1731 a party of settlers from the Canary Islands made it the first chartered civil settlement in what is now Texas. The mission church that became the Alamo is the building most people come for. But the five missions together are a World Heritage Site since 2015, the first in Texas, and the people who built and lived in them were mostly Coahuiltecan, not Spanish. The city has been Spanish, Mexican, Texan and American, and the food, the street names and the Sunday Masses still show it.
Three days is the right length because the center is compact and the sights are mostly small. The Alamo takes under two hours, the river loop an hour, and the real time cost is the four missions, which are spread over 6.5 miles of south side and deserve a whole day. A fourth day would go on Austin or the Hill Country, and both are an easy drive. San Antonio also rewards eating more than museum-going, so three days leaves room for lunches that last.
Day one is downtown: the Alamo, the River Walk, La Villita, the Spanish Governor's Palace and Market Square, ending under the San Fernando Cathedral light show. Day two is the four missions and a puffy taco on the way back. Day three is the Pearl, the river path, King William and the Tower of the Americas. Start on a Thursday, so the Pearl's Saturday market lands on day three and nothing you want is on a Monday, when the Palace, Ray's and the cathedral show are all closed. More on the country is on the United States page.