Split the regex into smaller parts to check each rule individually.

Here's the code for JavaScript. The same simple logic can be used for Ruby or any other language.

// Check the length
if (str.length >= 8 && validate()) {

}

// Check if all the characters are present in string
function validate(string) {
    // Initialize counter to zero
    var counter = 0;

    // On each test that is passed, increament the counter
    if (/[a-z]/.test(string)) {
        // If string contain atleast one lowercase alphabet
        counter++;
    }
    if (/[A-Z]/.test(string)) {
        counter++;
    }
    if (/[0-9]/.test(string)) {
        counter++;
    }
    if (/[!@#$&*]/.test(string)) {
        counter++;
    }

    // Check if atleast three rules are satisfied
    return counter >= 3;
}

Split the regex into smaller parts to check each rule individually.

Here's the code for JavaScript. The same simple logic can be used for Ruby or any other language.

// Check the length
if (str.length >= 8 && validate()) {

}

// Check if all the characters are present in string
function validate(string) {
    // Initialize counter to zero
    var counter = 0;

    // On each test that is passed, increment the counter
    if (/[a-z]/.test(string)) {
        // If string contain at least one lowercase alphabet character
        counter++;
    }
    if (/[A-Z]/.test(string)) {
        counter++;
    }
    if (/[0-9]/.test(string)) {
        counter++;
    }
    if (/[!@#$&*]/.test(string)) {
        counter++;
    }

    // Check if at least three rules are satisfied
    return counter >= 3;
}