.."Triad" by David Crosby.

I want to know how it will be
Me and her or you and me
You both stand there with your long hair flowing
Your eyes alive, your minds are still growing
Saying to me what can we do now that we
Both love you -- I love you too
I don't really see, why can't we go on as three
You are afraid, embarrased too, no one has ever
Said such a thing to you
Your mother's ghost stands at you shoulder
A face like ice a little bit colder
Saying to you
You can not do that it breaks all the rules
You learned in school
I don't really see, why can't we go on as three
We love each other it's plain to see
There's just one answer comes to me
Sister lovers -- water brothers
And in time maybe others
So you see what we can do
If we try something new - if you're crazy too
I don't really see, why can't we go on as three

Not really wife-swapping, but close enough.

… Let's Invite Them Over by George Jones & Melba Montgomery (a fantastic record, BTW)

I know why you're lonely and I know why you're blue
You're lonesome to see him and you long to see her too
We're not in love with each other we're in love with our best friends
So let's invite them over again.

We talk it over and over and we know it's not right
We should stay away forever but we're lonesome each night
Yes, we stay away for awhile but we know in the end
We'll invite them over again.

--- Instrumental ---

We talk it over and over and we know it's not right
We should stay away forever but we're lonesome each night
Yes, we stay away for awhile but we know in the end
We'll invite them over again...

R.e.m. track "Fretless" was originally called "Benjamin Caroline triangle" and i've always assumed it was about a person in a unorthodox three way relationship.

He's got his work and she comes easy
They each come around when the other is gone
Me, I think I got stuck somewhere in between
I wouldn't confide in the Prodigal Son
The die has been cast, the battle is won
The bullets were blanks, a double aught gun
I couldn't admit to a minute of fun

They come and they come and they come and they come
I accepted with a gentle tongue
No words spoken, no need to speak

Take it, stomp twice, ring the bell
Tether that ring and phrase
Enough with the rifle and talk already
We all know what it means
Take this conversation to your great divide
I can only swallow what I ate
And I don't hate him
And I don't hate her

They come and they come and they come and they come
I accepted with a gentle tongue
No heart broken, no need to speak

(Don't talk to me)
Don't talk to me about being alone
(Don't talk to me)
Don't talk to me about being alone
(Don't talk to me)
Don't talk to me about being alone

Reach for each other before you leave
Reach peace with an E-A-C
Don't threaten me with a gentle tease
Don't threaten me with angry
Please, please, please
Don't try to tell me what I am

They come and they come and they come and they come
I accepted with a gentle tongue
No words broken, no need to speak

(Talk to me)
Don't talk to me
(Talk to me)
Don't talk to me

Here you go ...

which I can't no matter how hard I try

The suburbs they are dreaming
They're a twinkle in her eye
She's been feeling frisky since her husband said goodbye
She wears a low cut T-shirt
Runs a little B&B
She's most accomodating when she's in her lingerie

Wife-swapping is your future
You know that it would suit ya

Yes They're stereotypes
There must be more to life
All your life you're dreaming
And then you
Stop dreaming
From time to time you know you should be
Going on another bender