That's because the Multi-Node Tree Picker stores only the Id's of the selected nodes. Depending on how you're storing the nodes, XML of CSV, you could do one of the following:

CSV:

@using umbraco.MacroEngines
@inherits DynamicNodeContext
@if (Model.HasValue("selectedNodes")
{
    string[] ids = Model.selectedNews.Split(',');
    dynamic nodes = new DynamicNodeList();
    
    foreach (string id in ids)
    {
        var node = Library.NodeById(id);
        if (node.Id != 0)
        {
            // If the node exists
            nodes.Add(node);   
        }   
    }

    foreach (var group in nodes.InGroupsOf(3))
    {
        <div class="parent">
        @foreach (var node in group)
        {
            <div><a href="@node.Url">@node.Name</a></div>
        }
        </div>
    }
}

XML:

@using umbraco.MacroEngines
@inherits DynamicNodeContext
@if (Model.HasValue("selectedNodes")
{
    dynamic nodes = new DynamicNodeList();
    foreach (var item in Model.selectedNews)
    {
        var node = Library.NodeById(item.InnerText);
        if (node.Id != 0)
        {
            // If the node exists
            nodes.Add(node);
        }   
    }
    
    foreach (var group in nodes.InGroupsOf(3))
    {
        <div class="parent">
        @foreach (var node in group)
        {
            <div><a href="@node.Url">@node.Name</a></div>
        }
        </div>
    }
}