My son who is only 36 months old has a very fast heart beat. His stool content partial blood strain after defication.?

Fast heart beat. The fast heart beat and blood in his stool may be related or two very distinct and separate signs. I would recommend that you take him to his pediatrician and have him evaluated for his bloody stools, and to determine if he has a normal, but fast, heart rhythm (due to dehydration, anemia, stress, etc.) versus a truly abnormal heart rhythm (much less common).
Check his Hemoglobin. If he is passing several stool with blood he can be anemic and his fast heart rate secondary to the anemia. Other thing to do an ECG at his pediatrician office.