<?xml version="1.0" encoding="utf-8" ?>
<Ref1 Att11="XXX" Att21="YYY" Att31="ZZZ">
 <Ref2 Att12="AAA" Att22="BBB" Att32="CCC">
 <Ref3 Att13="111" Att23="222" Att33="333"/>
 </Ref2>
</Ref1>

public class Ref1
{
    public string Att11 { get; set; }
    public string Att21 { get; set; }
    public string Att31 { get; set; }

    public List<Ref2> ref2;
}

public class Ref2
{
    public string Att12 { get; set; }
    public string Att22 { get; set; }
    public string Att32 { get; set; }

    public Ref3 ref3;
}

public class Ref3
{
    public string Att13 { get; set; }
    public string Att23 { get; set; }
    public string Att33 { get; set; }
}