JSFiddle - React, Tailwind, and code Playground

HTML

ФИО<input id='fio' onkeyup='checkParams()' /><br />
ПОЧТА<input id='email' onkeyup='checkParams()' /><br />
ТЕЛЕФОН<input id='phone' onkeyup='checkParams()' /><br />
<input type='button' id='submit' value='ОТПРАВИТЬ' disabled>

JavaScript

function checkParams() {
    var fio = $('#fio').val();
    var email = $('#email').val();
    var phone = $('#phone').val();
    
    if(fio.length != 0 && email.length != 0 && phone.length != 0) {
        $('#submit').removeAttr('disabled');
    } else {
        $('#submit').attr('disabled', 'disabled');
    }
}