help with my muscovy behaviour

hi, i have just introduced a female duck to a pair about the same size and age, about 5 months old. the pair have just start a new behaviour of thrusting their necks forward in rhymn with each other, is this an aggressive behaviour, it looks quite funny. the new duck does it sometimes. the two duck are not getting on the first duck keeps on chasing her away from the pond and nipping at her, the new duck has been here for 1 week, will this ever settle down. they dont seem to have made contact yet or caused any injury, they all sleep together.  

ok, i will try and get some tomorrow, if the rain holds off. do you think they will except her or should i think about getting her a drake and moving the pair.

thanks for your help, it is really good to get some advice from someone who has experience. i will try and get some photo when the rain stops!!