you have to finalize your Residence permit within these 3 months. If your RP isn't issued yet upon completion of 3 months period from date of entry, your work visa will be expired.
push your employer to fit this timeframe.. although the RP can be finalized within 1 week, some companies keep postponing this issue..
.. once your RP is issued, it's valid for 5 years stay in total. But first, your RP will be issued only for 1-2 years, and then renewed every year. Once 5 years are expired, you will have to "renew" your Work visa.
Long story short, don't worry.. everything is all right. Just make sure your RP is issued on time..