Validating multiple form fields in one function

I have the following code that is designed to validate a for field when entering values ​​into the field (red for invalid and green if the field becomes valid):

function FormValidation(){
    var fn=document.getElementById("firstName").value;
    if(fn == ""){
        document.getElementById("firstName").style.borderColor = "red";
        return false;
    }else if (/^[0-9]+$/.test(document.getElementById("firstName").value)) {
        document.getElementById("firstName").style.borderColor = "red";
        return false;
    }else if(fn.length <=2){
        document.getElementById("firstName").style.borderColor = "red";
        return false;
    }else{
        document.getElementById("firstName").style.borderColor = "#679f30";
    }
}

      

        
        
        
      

    

These checks will also be required for other fields.

I tried a for loop but it doesn't work to validate all forms like the above code for one field:

function FormValidation(){
    var array = ["firstName", "middleName", "lastName"];
    for(i=0; i < array.length; i++){
        var fn=document.getElementById(array[i]).value;
        if(fn == ""){
            document.getElementById(array[i]).style.borderColor = "red";
            return false;
        }else if (/^[0-9]+$/.test(document.getElementById(array[i]).value)) {
            document.getElementById(array[i]).style.borderColor = "red";
            return false;
        }else if(fn.length <=2){
            document.getElementById(array[i]).style.borderColor = "red";
            return false;
        }else{
            document.getElementById(array[i]).style.borderColor = "#679f30";
        }
    }
}

      

        
        
        
      

    

So my question is, am I doing something wrong? Or is there a way to validate more than one form in one function without having to write the same code over and over.

Any help is greatly appreciated!

Thanks Al