Herbert Birdsfoot:

Oh, I can only sing the high part.
It's the only part I know.
And when I sing the high part,
This is how I go:
La, la-la-la-la-la-la, la
La, la-la-la-la-la-la.

Ernie:

I can only sing the middle.
It's the only part I know, I know.
And when I sing the middle part,
This is how I go:
La, la-la-la-la-la-la, la
La, la-la-la-la-la-la.

Bert:

I can only sing the low part, low part,
It's the only part I know, I know.
And when I sing the low, low part,
This is how I go.
(That's beautiful, Bert!)
La, la-la-la-la-la, la-la, la-la,
La, la-la-la-la-la-la.

Sung by all 3 together:

When we put them all together
I am sure you will agree, agree,
The whole sounds better than the parts
As you can plainly see.
La, la-la-la-la-la, la-la, la-la,
La, la-la-la-la-la-la, la la la la.