OBi2xx series devices can support 1 to 4 VoIP "lines" (ie numbers or accounts) but have have only 1 (OBi200 and Obi212) or 2 (OBi202) ports to connect phone or fax machine.
If you require all 3 of your home, office and fax end-points to be truly independent and all able to be active on calls at the same time then you need at least 3 phone ports. Another factor is the physical layout and wiring. Is it easier for you to have two OBi200s with ethernet cabling or OBiWiFi adaptor, each close to the phone or fax than an OBi202 and phone cabling to the end-points?
You may be able to put both office phone and fax on a single phone port even if they have separate incoming numbers. If your fax machine supports "distinctive ringing" the OBi can be configured that calls to the fax number have a different cadence which the fax machine recognises and picks up. Of course you can't have fax and phone in use simultaneously.
So 2 ports or 3?
Then how to achieve those 2 or 3? A single OBi202 or a pair of them or a 202 plus a 200 or 3 off 200s? Or even a pair of 202s which gives you scope for expansion.
And if you want to retain or ever add a true POTS line or a VoIP line with a POTS style presentation (such as your Comcast one) the the recently available Obi212 may be of interest. It has one PHONE jack and one POTS jack.