Thêm code css này là được nha, tại menu cấp 3 mặc định flatsome cực xấu CODE CSS menu 3 cấp trên theme Flatsome

.nav-dropdown>li.nav-dropdown-col{display:block}
.nav-dropdown{border:1px solid #ddd; padding:0}
.nav-dropdown .nav-dropdown-col>a, .nav-dropdown li a{font-weight:normal !important; text-transform:none !important;font-size:15px; font-weight:500}
.nav-dropdown .nav-dropdown-col>ul li:hover{background:#db0000}
.nav-dropdown-default>li:hover>a, .nav-dropdown .nav-dropdown-col>ul li:hover>a{color:white}
.nav-dropdown-default>li:hover{background:#fbae3c}
.nav-dropdown-default>li>a{border-bottom:0 !important}.nav-dropdown-has-arrow li.has-dropdown:before{border-width: 10px;
    margin-left: -10px;}
.nav-dropdown .nav-dropdown-col>ul{border: 1px solid #d2d2d2;margin-top:-40px;box-shadow: 2px 2px 5px #828282;display:none;position: absolute;
    left: 100%;z-index: 9;background: white;min-width: 240px;}
.nav-dropdown>li.nav-dropdown-col{width:100%;border-right:0}
.nav-dropdown .nav-dropdown-col>ul li a{padding:10px;  text-transform: none;color:black}
.header-nav li.nav-dropdown-col:hover >ul{display:block !important}