Each device you want to use will need to be Bluetooth enabled - either built in capability like that of some mobile phones and PDA`s or with an adapter, which come in various forms.
For the two PC`s you can get simple usb dongles or pcmcia cards if you are using a laptop. Prices have dropped quite a bit recently, a search on Google should help find a suitable supplier.
Your mobile may present more of a problem. Some, like the Nokia 6310i or the Eriksson T68i, have Bluetooth as standard but most phones do not. The adapters available for phones like the Nokia 6210 are pricy and hard to find.
That`s the outline of how but now why?
Bluetooth is a PAN network, the P stands for personal and it means it! Range is pretty much limited to devices in the same room, OK for linking a mobile and a laptop or maybe keeping folders in sync with a PC but quite restricting for any proper network.
Second, setting up a Bluetooth device can be quite a trying exercise. It rarely works the way you would expect it to first time!
If you have the patience and are determined I find that it works well between my laptop and 6310i phone for modem use. The effect on the phones battery life is much less than using IR.
The TDK usb adapter on one of my desktop systems suffered from poor drivers causing system instability but when it did work it was find for keeping mobile phone books updated and/or backed up.
As I have both WiFi and cabled ethernet available I have`nt spent too much time experimenting with all the networking features available with Bluetooth, so I won`t make any comment on this.