Are people with open asd more likely to get sick?

Possibly. This depends on the size of the asd, age of the patient, and whether the ASD has been repaired. Small asds may have no effect while a large ASD that has been left untreated for a long time (e.g. 4o years etc) can certainly lead to more illnesses.
Possibly . Unlikely to be a measurable difference except for bacterial endocarditis paradoxical, embolism, and if large late heart failure.
No. An ASD rarely causes any symptoms in children. It does not affect the ability of the body to fight infection.