public class Extended_Model
    {
        public IEnumerable<Model1> Models1 { get; set; }
        public IEnumerable<Model2> Models2{ get; set; }
        public IEnumerable<Model3> Models3 { get; set; }
        public IEnumerable<Model1_Model2> Rel_Model1_Model2 { get; set; }
        public IEnumerable<Model1_Model3> Rel_Model1_Model2 { get; set; }
    }

@Html.RenderAction("Create", "Model1");

@Html.RenderAction("Create", "Model2");

@Html.RenderAction("Create", "Model3");

@model Model1
<!--Whatever HTML code you want to edit/create your Model1-->

@model Extended_Model
@using (Html.BeginForm())
{
    @Html.AntiForgeryToken()
    @Html.EditorFor(m => m.Models1)
    @Html.EditorFor(m => m.Models2)
    @Html.EditorFor(m => m.Models3)
    @Html.EditorFor(m => m.Rel_Model1_Model2)
    @Html.EditorFor(m => m.Rel_Model1_Model2)
    <input type="submit" value="Save" />
}