Water Heater Exhaust - how to connect the 3 inch hood to four inch exhaust pipe
It may not be pretty but there isn't anything wrong with using aluminum tape on this. However, there are other options.
There are a number of reducers and elbows that will fit:

This is a 3" to 4" adaptor that should get you started and can be combined with a flexible elbow:  


These are American Metal Products available at HD. You may have to do some adjusting - ductwork is as much art as science. If these particular items don't work for you there are others out there.