How to detect an read buttons pressure on touchscreen
The fillRoundRect function format is (initial_Xpoint,initial_Ypoint,length,width,corner_radius,color)

Therefore, you already have the information about the location of the button itself. Next, check if the x and y value is within the button area.

void setup() {

  tft.begin();
  tft.fillScreen(BLACK);

  tft.fillRoundRect (12,15,30,80,5,RED);  //button1
  tft.fillRoundRect (50,15,30,80,5,RED);  //button2
  tft.fillRoundRect (88,15,30,80,5,RED);  //button3


  delay(500);
}

void loop()
{
  int x, y, pressedbutton=0;                         

  if(ts.dataAvailable())         
  {
    ts.read();                //-----------&gt; this is the whole thing      
    x = ts.getX();                  
    y = ts.getY();                  

    if ((x&gt;=12) &amp;&amp; (x&lt;=12+30) &amp;&amp; (y&gt;=15) &amp;&amp; (y&lt;=15+80)) pressedbutton = 1; else
    if ((x&gt;=50) &amp;&amp; (x&lt;=50+30) &amp;&amp; (y&gt;=15) &amp;&amp; (y&lt;=15+80)) pressedbutton = 2; else
    if ((x&gt;=88) &amp;&amp; (x&lt;=88+30) &amp;&amp; (y&gt;=15) &amp;&amp; (y&lt;=15+80)) pressedbutton = 3; 
  }

  ...
  ...
  ...
 }