I have a good friend who's as kind as can be
I needed one so he gave one to me
If he gave me another, then he'd be a fool
The answer is found in a casual pool

The first three lines refer to a transplant.
The last is a kidney-shaped ("casual" not Olympic) pool.

(Adam E.:If the friend's name is Mulligan, one often grants a Mulligan in
a casual game of pool.)

See the twins and the triplets and you're overjoyed
But a laugh or a smile you choose to avoid
The twins act as single, at least they do try
The thing is as funny as a poke in the eye.

The first two lines refer to the poker hand.
The last two refer to that horrid television show with the Olsen Twins.

Several Greek letters
standing in lines
They got up, one pulls oars
And they smell oh so fine.

Described as Rhos,rows,rose,rows,rose.

The loudest you can make your voice
Mix it up, you've made a choice
Now put it back, this ain't a joke
You do it wrong, you have a smoke.

Top of the voice, mixed up gives opt.
Be careful when putting the letters back together, or you get pot.

Helen and Amanda: Vote / Veto / Tove

In what context are the words "wide" and "sound" opposites?

You can be wide awake or sound asleep

Helen and Amanda: Geography (a "sound" is a narrow passage)

The committee members,
listless and lank
picked up their tools
and drilled holes in a plank.

Described as board and two meanings of bored

Cowboys' horses, bucking wild
Across a river they slowly filed
To the scene of John's crime
For the thirty-eighth time.

What can get through it
Invisible, with a lock?
Everything and nothing
(except, maybe, a rock).

Ideally, windows in good homes are invisible.

It helps her to breathe
And not to conceive

The chest-muscle and birth-control device.

anything at all
captured at sea
null 'n' void
instructed at tea.

Aught: Anything at all
Caught: C + aught
Naught: N + aught
Taught: T + aught