You need to separate your goals out.
- File Server
- Turn PC off
If you turn a file server off, you can not connect to the drives. Period. No way around it, but your PC and your file server do not have to be the same thing.
No problem, just don't use your PC as a file server and you can turn it off, leave it on or bring it to a friends house.
This is a relative term, define fast. 100Mbps? 1Gbps? 10Gbps? Your budget sets the limit! with NAS (Network Attached Storage) these are essentially all-in-one file servers that you can plug in and go wired or wireless.
As far as your OS choices, the NAS is the better choice too, as sharing each drive / partition out for all 3 OS's and not porking the ACLs could be fun. With a stand alone file server you don't have to worry about that.
For speed, if you want 100MBps, thats 800Mbps, or less than 1Gbps. bits per second is what most throughput metrics use, so you need a drive and a network that can support 1Gbps to realize that goal (Should not be a big deal currently).
As for accessing the drive like any other drive, as long as you map the share thats not a problem, here is a share I have mapped:
Almost everything will see that the same as a regular drive. There are rare exceptions from especially shoddy software.
"Porking the ACLs" I mean screwing up who has access to it. If you are sharing it through a workgroup and again through SAMBA between Linux and Mac there exists the chance one will try to change the owner or access groups. Thus when you boot back to another OS your permissions and access are broken.