hi i got alternator problems i think ?

i have built my own wiring loom for the car and charging circuit and think i might have made a mistake ?

the brand new alternator wont start to charge till i give it a good rev to 1400rpm is ,everytime i start the car ? once it starts to charge it seems fine ,this fault was the same with the old alternator ?

so question is what are the 3 terminals for on a 2wd cosworth alternator

thanks