My fianc will go up to 3 days without going poop. and she says that she rarely has gas. i feel like this is not normal at all. any suggestions?

No: many people have different habits. you may get answer from example in our Wright's physiology book. In London, one person did not go to bathroom for 1 year. There was another person who checked himself in all the 30 toilets on his way to work.

Not that unusual: Not everyone has a bowel movement every day. As long as she is not in discomfort and this is a normal pattern for her I wouldn't worry. If she rarely has gas, then consider yourself lucky.