ELP opened for Jethro Tull on one of concerts I went to.
Procul Harum on another and I can’t recall the third openers.
All at Merriwether Post Pavilion.
Also saw ELP there, Brain Salad Surgery days. Great concert.
My favorite concert, prior to 2019's Crossroads Guitar Festival was Spyrogyra in El Paso. Due to a change in venue, my front row, balcony seats were converted to THE front row. It wasn't until years later, my wife said she hated it...lol.