Orderby and thenby using LINQ

Question

I have below table for categories.

tbCategories

CatgID        CatgName
------        --------
  1           Catg1
  2           Catg2
  3           Catg3
  4           Catg4

Now I have items table which is categorized linking to above categories

tbItem

ItemID            ItemName           ItemCatg         Priority
------            --------           --------         --------
  1                Name 1               1                 1
  2                Name 2               1                 2
  3                Name 3               2                 1
  4                Name 4               3                 3
  5                Name 5               4                 2
  6                Name 6               3                 1

Priority

1 - High
2 - Medium
3 - Low

Now when I display the item data, I want the to first display items with category Catg1, Catg2 and Catg4 based on their priority, then display items with Catg3 based on their priority.

I had tried below LINQ expression, but I did not succeed with that. My knowledge on LINQ or SQL is not too strong but that's how much I could reach.

model.items = db.tbItem.OrderBy(p=>p.ItemCatg).ThenBy(p=>p.Priority).ToList();

Could someone please show me the right way to achieve this functionality?

UPDATE

I would like to achieve below combination while displaying.

ItemID            ItemName           ItemCatg         Priority
------            --------           --------         --------
  1                Name 1               1                 1
  3                Name 3               2                 1
  2                Name 2               1                 2
  5                Name 5               4                 2
  6                Name 6               3                 1
  4                Name 4               3                 3

Answer 1

The following is a trick I've used in the past. You're basically relying on the result of the comparison ordering things first by the non-3 categories (i.e., the false equates to 0, the true to 1) then by the priority, then by the category.

tbItems.OrderBy(p=>p.ItemCatg == 3).ThenBy(p=>p.Priority).ThenBy(p=>p.ItemCatg).ToList();

Here's a .NET Fiddle example.

Here's the output from the example:

ItemID: 1    ItemName: Name 1    ItemCatg: 1    Priority: 1
ItemID: 3    ItemName: Name 3    ItemCatg: 2    Priority: 1
ItemID: 2    ItemName: Name 2    ItemCatg: 1    Priority: 2
ItemID: 5    ItemName: Name 5    ItemCatg: 4    Priority: 2
ItemID: 6    ItemName: Name 6    ItemCatg: 3    Priority: 1
ItemID: 4    ItemName: Name 4    ItemCatg: 3    Priority: 3