Set minimum required length of a text input field using javascript?

I'm not entirely sure about what you want; is this close to what you are looking for?

You probably did this and didn't include it in your snippet, but we need this to run each time the form is edited. We add an event listener to the input.

const input = document.getElementById('npo-registration-number');
input.addEventListener('input', () => {
    // set the border to red if the value is < 7 characters
    if (input.value.length < 7) {
        input.style.borderColor = 'red';
        return;
    }
    // otherwise, set it to green
    input.style.borderColor = 'green';
});

This fixes an issue: you do not set the color of the border to red unless the value of the form is an empty string. Rather, we want the border to be red whenever the input length goes below seven.

It's because of your condition

if (v3.value == "") {
            v3.style.borderColor = "red";
            flag3 = false;
        }

When you start deleting characters in input, your input is not becoming red again because It's red only when there are no characters and it change to green after 7 characters inputed.

You need to capture the input event so js can evaluate and update. https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/input_event

     const ev3 = document.getElementById("npo-registration-number");
     let flag3 = true;
     ev3.addEventListener('input', (e) => {
       const val = e.target.value;
       if (val.length === 7){
                ev3.style.borderColor = "green";
                flag3 = true;          
        }
        else {
           ev3.style.borderColor = "red";
           flag3 = true;   
        }
     });

input {
  outline: 0;
  border: 1px solid;
}

<input id="npo-registration-number" type='text'>

You can just use button that will run this function for it, like send or post buttons(many sites use this method).

function click(){
  v3 = document.getElementById("npo-registration-number");
  flag3 = true;

  if (val >= 3 || val == 0) {
        if (v3.value == "") {
            v3.style.borderColor = "red";
            flag3 = false;
        }
        else if (v3.value.length === 7){
            v3.style.borderColor = "green";
            flag3 = true;
        }
   }
}