Be more proactive in your design, use the internal pullup resistors you know the pins are not 'floating'.
byte pin1 = 2;
byte pin2 = 3;
byte pin3 = 4;
void setup(){
pinMode (pin1, INPUT_PULLUP); // turn on internal pullup resistor wire button between pin & Gnd
pinMode (pin2, INPUT_PULLUP);
pinMode (pin3, INPUT_PULLUP);
void loop()
{
button1 = digitalRead(pin1);
button2 = digitalRead(pin2);
button3 = digitalRead(pin3);
if ((button1 ==LOW) && (button2==LOW) && (button3==LOW)) {
//all 3 button pressed same time
//do something here
}
}