public class AccessRole
{
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
        public virtual ICollection<AccessRoleRight> AccessRights { get; set; }
}

    
public class AccessRoleRight
{
        public virtual int Id { get; set; }
        public virtual AccessRight AccessRight { get; set; }
}

public class User
{
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
        public virtual ICollection<AccessRole> Roles { get; set; }
}

var lst = Roles.SelectMany(r => r.AccessRights).Select(s => s.AccessRight).ToList();