That would entirely depend on the ability to access the third floor to pull the fibre. It is more likely the ONT will be installed at a lower level, and existing Ethernet or coax will be used to distribute internet to the other floors.
PureFibre arrives to a single point in your dwelling. It then, as @giantbrownguystates, converted to an Ethernet signal where it is distributed around the home using traditional network devices. Unless there is an existing conduit from the communication cabinet to the third floor, Telus will use existing wiring to transmit the Ethernet signal to your office.
I was part of one of the original city wide installs, and they went to my garage, as it was the easiest location. In my new place, they ran the fibre along the outside of the house to the living room. You might be able to get them to do it if there is easy access, but it might cost you as well. I believe Telus likes to go to the route of least inconvenience, and that typically is the shortest run from the telecommunications box outside your house. That said, it also needs to be near your modem, so if you modem has to be in the office, that might help your case.
I have a new house and when they came to do the fibre optic install, they said they can't lay the wire past the telus utility box. Its copper only after that point. They installed my ONT box dead square in the middle of my living room wall, and then a huge back up power box right next to it for back up battery power for my phone in case the power goes out. Its a total eye sore in a new house to have two huge boxes hanging off you walls. At night they flicker alot of green status lights which get really annoying if you want to take a nap. I really wish Telus was willing to lay wires to a more discreet spot of my house, totally regret not going with SHAW.
I had a lot of concerns getting PureFibre installed. My concerns were about equipment location, drilling of holes in the wall and aesthetics of what things would look like when they left.
To address this what I did (with some difficulty) was to do the fishing (i.e. laying a feed piece of rope from the outside of the house to the desired installation location) within my walls BEFORE TELUS came. This was I was able to ensure that the fibre was pulled to the location where I wanted the ONT and modem to be located. If you aren't confident doing the fishing hiring an electrician or a communications installer beforehand might be an idea. If you want to try this using your own fishing gear, I would recommend the longer fibreglass fishing sticks (the ones I used were almost a metre long). The coiled fibreglass fishing gear often uncoils in the wall. If you have conduit installed, the coiled gear might work.
With the fishing done beforehand, the installation was easier and it ended up being in the location where I wanted the equipment.
When I talked to the TELUS technician he said that he had only seen three instances where people had done the fishing prior to the technician coming to do the installation. Having the line fished through prior to the technician's arrival did help the process a great deal!