Need help with double join in LINQ query

Can someone help me with a double left join in LINQ?
Or perhaps suggest a better way. The goal is to get a list of the stores an appUsers has the authority to access. An appuser record's [Stores] column may contain a store number or it may contain a groupname. If a groupname, the storegroups table contains a list of stores in the group. Then I want to return the store number and the name of the store.
Give these tables:

>> Truncated for clarity <<
CREATE TABLE [dbo].[StoreGroups](
	[GroupName] [nvarchar](30) NOT NULL,
	[Store] [int] NOT NULL,
	[Client] [smallint] NOT NULL CONSTRAINT [DF_StoreGroups_Client]  DEFAULT ((1)),

CREATE TABLE [dbo].[Stores](
	[Client] [smallint] NOT NULL CONSTRAINT [DF_Stores_Client]  DEFAULT ((1)),
	[Store] [int] NOT NULL,

CREATE TABLE [dbo].[AppUsers](
	[UserID] [nvarchar](20) NOT NULL,
	[Password] [nvarchar](50) NOT NULL,
	[AuthorityLevel] [smallint] NOT NULL,
	[Stores] [varchar](30) NULL,
	[Area] [varchar](50) NULL,

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I have this SQL query:

Select * 
from appusers au left join storegroups sg on au.stores = sg.groupname 
left join stores s on (isnumeric(au.stores) = 1 and au.stores = s.store) or sg.store = s.store

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Here is how I tried it:

  storeString = (from au in xm.AppUsers
                           join sg in xm.StoreGroups
                           on au.Stores equals sg.GroupName into JoinedGroups
                           from jg in JoinedGroups.DefaultIfEmpty().....

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but I got stuck with the second left join.