My wife is 3 month pregnant and she is suffering by fast heart beating many times a day.

Arrhythmia? Pregnancy can unmask arrythmias...Best get to get her checked out by a physician. Holter monitor & echocardiogram are two probable tests that will be performed. Good luck.
Fast Heart Beat. Either she might be having anxiety attack or tachy arrythmias. As she is having these epsiodes, it can be diagnosed through 24 hours holter monitor. Please see your pcp or cardiologist.