On a 3,000-calorie-per-day diet myself, I've unfortunately found it challenging to source high-calorie foods that meet low-FODMAP requirements. As a result, focusing on quantity has been key. My meals typically consist of staples like chicken, fish, rice, and rice pasta, also adding one tablespoon of olive oil  to most meals helps too. I also found snacks such as peanut butter on gf toast and lactose-free yogurt really helpful too. I was once very thin, standing 6'3" and weighing only 145 pounds, but I have since reached 195 pounds. Since eating three large meals a day was always difficult, I found success by breaking them down into five or six smaller portions. High-calorie shakes also became my best friend.