Can having an atrial septal aneurysm with a hole in it cause brain aneurysms which i have 3. also is it safe for me to donate plasma?

Possibly: I cannot answer the plasma question but if you have an atrial septal defect then yes, any infected blood clots or material that gets into your veins could get around your lungs and cause it a brain aneurysm (paradoxical embolization). This should be discussed with your cardiologist to see if an atrial septal occluder device is needed.

Unlikely: Unless the cerebral ones are septic from cardiac infection and embolization and cerebral abscess. Ask the local blood bank. I doubt they will want to possibly test your reserve - even with a good blood type.