How to use 3 digital input pins on Arduino Uno for frequency inputs and total the output?
Assuming that only one sensor will trigger each time then (as jsotola suggested) tie them together and connect the common output to an interrupt pin. Arduino UNO has interrupt functionality on pins 2, 3.
As an example for counting the pulses using pin2 on falling edge (modified from https://www.arduino.cc/reference/en/language/functions/external-interrupts/attachinterrupt/):
const byte interruptPin = 2;
volatile int numPulses = 0; // change type of variable according your needs

void setup() {
    pinMode(interruptPin, INPUT_PULLUP);
    attachInterrupt(digitalPinToInterrupt(interruptPin), sumPulses, FALLING);
    Serial.begin(9600);
}

void loop() {
    Serial.println(numPulses);
    delay(500);
}

 void sumPulses { // keep the ISR as short as possible
    numPulses++;
 }