Since you asked,
Droneklax --
Your Question: "Was my share $150/3 or $150/2?"
My Answer: $150/2
I agree with the various posts above and would like to offer my perspective on it. In this situation you offered and they agreed to share their limousine and the $150 fare with you. Based on your conversation with the couple, I really feel they were correct in expecting the $75 for sharing their limousine with you. By the way, the gratuity on a $150 bill, if it were not included, would be an additional $25 or so. They gave up any and all privacy they would have had and that may have been priceless. To avoid any misunderstanding, I never assume anything. It's necessary to be pro-active. I always spell out the terms, dollar amount, or any expectations.
To this day, I am mad. Not only because I was a poor student and they were pretentious jerks who obviously had money yet argued over $25, but also because I felt I had been taken for ride.
The fact that they had money or "argued" over $25, I feel, doesn't justify your characterization of them as "pretentious jerks." Believe me, I've driven many in my time, some of them even big name celebrities.

I believe this couple simply wanted and expected you to live up to your end of the agreement. They didn't approach you, you approached them. They were one half of the equation, I feel.
I'm sorry for any misunderstanding you had and most sorry that "To this day" you are still mad and it is bothering you. I hope our various posts offer you the insight and understanding to move on. I also hope that the limousine trips you've had since then have been enjoyable ones.
Happy travels and have a nice day!