Selecting The Same Column Twice In The Same Where Clause

Hi :
From a crystal report i get a list of employee firstnames as a string
into my store procedure. Why is it comming this way ? hmmmmmm it's a
question for me too.
ex: "e1,e2,e3"

here are my tables

tblProjects
ProjectId
1
2
3

tblEmployee
employeeId FirstName
1 e1
2 e2
3 e3

tblProjectsToEmployee
ProjectId employeeId
1 1
1 2
1 3
2 1
2 2
3 1
3 2
3 3
4 1
4 3

i need to find out the project ids all 3 of these employees worked on.
so the out put i need is

projectId
1
3
How can i get it ????????????

now i can use replace command to format it to a OR clause or AND
clause
SET @string= 'employeeId =' + '''' + REPLACE('e1,e2,e3',',',''' OR
employeeId = ''') + ''''
some thing like this.

OR clause will give me all 4 projects.
in('e1','e2','e3') will give me all 4 projects.
of cause AND command will not give me any.

other method i tried was adding the employee table 3 times into the
same SQL string and doing some thing like

WHERE (empTable1.Firstname ='e1' AND (empTable1.Firstname
in('e2','e3'))
AND (empTable2.Firstname ='e2' AND (empTable1.Firstname in('e1','e3'))
AND ...

and goes alone. this gives me some what i needed. but it's a very
messy way of doing it, because i get a comma seperated string
parameter i have to construct the sql string on the fly.

any help or direction on this matter would greatly appreciated.

thanks
eric