The Everest Three High Passes trek requires trekkers to have a basic level of fitness stamina. For that, trekkers need to go through training a couple of months or weeks ahead of the trek.
Training should include strength and endurance-building exercises, cardiovascular or aerobic activities, and mental exercises. You can get help from a trainer or train yourself for the trek. To remain prepared, you can take practice hikes or learn more about the trek from past trekkers along with the internet.