Try using just the one println and see if it helps.
oid loop() {
milli_time = millis();
voltTC = 5.000 * analogRead(A0) / 1024.000;
voltTH = 5.000 * analogRead(A1) / 1024.000;
voltpH = 5.000 * analogRead(A2) / 1024.000;
Serial.print("DATA,TIME,");
Serial.print(milli_time);
Serial.print(",");
Serial.print(voltTC);
Serial.print(",");
Serial.print(voltTH);
Serial.print(",");
Serial.println(voltpH); //last serial command uses println
delay(1000);