menu not showing on mobile view

It’s because in your themes style.css there is a media query written on line no. 12401

@media (max-width: 991px)
.header__wrapper .main-menu3 {
     display: none;  
}

first, comment on this line now you can see your menu shown in your mobile view also.
To toggle this when clicking on three lines you have to add a little jquery code:

$('.button-show').click(function(){
    $('.main-menu3').toggle();
});

or you can achieve the same thing in one more way:

$('.button-show').click(function(){
    $('.main-menu3').toggleClass('hide_menu');
});

Now in your theme CSS write follwing css:

.hide_menu{
    display:none;
}

let me know if you are having any more doubts.
Thank you
Regards,
Ravi Thakkar