SQL Server 2014 :: Getting Unique Number For Groups Of Records?

I'm trying to do the following and haven't been able to figure it out.

Say there's a table with these records:

Col1 Col2 Col3
a b c
a b c
a b d
e f g
e f g

I want to generate a number that represents the groups of columns like this:

Col1 Col2 Col3 MyNumber
a b c 1
a b c 1
a b d 2
e f g 3
e f g 3

So that each grouping gets its own identifier. I've tried this:

SELECT Col1, Col2, Col3
row_number() OVER (PARTITION BY Col1, Col2, Col3
ORDER BY Col1, Col2, Col3) AS MyNumber
FROM MyTable

But I get this:

Col1 Col2 Col3 MyNumber
a b c 1
a b c 2
a b d 1
e f g 1
e f g 2