I'll add a bit more to what you already have, from memory:

Three little unexpected children
simultaneously the doctor brought us
and you can see there will be three for ever and
...

We do everything alike,
we dress alike
we look alike we walk alike we talk alike and what is more
we hate each other very much,
we hate our folks,
we are sick of jokes and what an art it is to tell us apart!

If one of us gets the measles
The second one gets the measles
Then all of us get the measles
And mumps, and flue

How I wish I had a gun
A widdle gun
I'd kill the other two and be only one!