JOIN QUERY between three tables

Firstly, in my copy of the Northwind database, downloaded directly from MSDN, the tables are called Employees, Orders and Customers. It is important to be precise.

The tables can be queried using JOINs - this CodeProject article explains the various types of join Visual Representation of SQL Joins[^]

Because you are trying to limit the information returned - "all employees that have customers" implies "do not show employees that do not have customers", an INNER JOIN would be appropriate - refer to the article to see why.
You don't actually need to join across the three tables because if you look at the Create script for the table Orders you will see

...
[CustomerID] [nchar](5) NULL,
...
ALTER TABLE [dbo].[Orders]  WITH NOCHECK ADD  CONSTRAINT [FK_Orders_Customers] FOREIGN KEY([CustomerID])
REFERENCES [dbo].[Customers] ([CustomerID])
GO
ALTER TABLE [dbo].[Orders] CHECK CONSTRAINT [FK_Orders_Customers]
GO

(In SQL Server Management Studio, if you right-click on a table name in the Object Explorer window, select "Script table as" then "CREATE to" then "New Query Editor Window" you can get to the information pasted above).
That basically means that if you do include a value for CustomerID on the Orders table then it must exist as CustomerID on the Customers table, otherwise that column will contain NULL.

So, to just list the Employees details that appear on the Orders table:

Select E.LastName, E.FirstName, E.Title
FROM Orders O
INNER JOIN Employees E ON O.EmployeeID = E.EmployeeID
WHERE CustomerID IS NOT NULL

That happens to be exactly the same as including Customers on another INNER JOIN i.e.

Select E.LastName, E.FirstName, E.Title
FROM Orders O
INNER JOIN Employees E ON O.EmployeeID = E.EmployeeID
INNER JOIN Customers C ON O.CustomerID = C.CustomerID

Again, if you look at the images on that article it will become clear why.

Translating that to MVC, Linq or whatever else you are trying to do is left as an exercise for you, as you have not provided enough information (nor effort) for me to help you with that.

Select E.LastName, E.FirstName, E.Title --,and some other employees fields
FROM Orders O
INNER JOIN Employees E ON O.EmployeeID = E.EmployeeID