Admission & retirement

My query is that if presently there are four partners in a firm,

three have to retire, and other three persons have to join the firm,

what would be the procedure ?

1. In one deed only should there be admission & retrirement or

2. would there be two deeds to carry out the same, one being Retirement and another for Admission (but in this case after retirement only one partner is left, which cannot be possible for partnership) 

Thanks in advance.