It's more about what features and specifications your NVR would require: if you are going to add "only" 3 cams, a "simple" NVR4xxx would suffice, but if you're aiming high (with all smart codecs, 320Mbps bandwidth, up to 12MP preview/playback), you're up for the NVR5xxx series, called not for nothing the "professional" series.
If you have picked the series, then you need to think whether (or not) you want onboard POE ports (which eliminates the use of a separate POE switch).
And off you go!
PS. do not forget to ask
@EMPIRETECANDY for a quotation
Good luck!
CC