alternator wiring problems

my alternator on my '97 megane classic 1.6e RT gave out the other day, so i bought a replacement renault 75A one from the internet. upon changing them i noticed that the old one has 3 wire connection points and the new only has 2. the car starts and works even with only 2 connected but since changing it the speedo has stopped working and the 'electrical component fault' light occasionally comes on. also there is a knocking noise coming from behind the dash board on the passenger side (like 6 knocks every 20 seconds or so). i cant find a replacement alternator with 3 connection points anywhere either. any ideas if its all related and how to sort it