Blood test showed I have low iron so I take iron tablets 3 a day but I get constipation dark green poo and now for the past days feel very sick…?

Not unusual. It's not unusual to notice changes of the color or consistency of your stool after starting an iron supplement. There are multiple types of iron supplements, some of what you may tolerate better than others. If you would continue to have issues with this, contact the doctor that prescribed this for you and see if he/she would prefer to switch it to another formulation.