How do I connect 3 wireless routers to make one... (by 2 users)

I have 3 wireless routers aquired through different service providers when I've signed up for their services.

1. Netgear DG834GT
2. Siemens Gigaset SE587 WLAN dsl
3. Thomson Speedtouch ST585 v6

I don't have the money to buy new products so I'm trying to connect my upstairs computer, wired to the Netgear, to my phone line which is downstairs. I also want to connect my XBox360 which is in another room.

I am unable to rewire or extend the phone point nor hardwire the routers.
I had the idea of connecting:
- the Siemens with its dsl plugged into the phoneline (downstairs)
- my computer with the Netgear (upstairs)
- the Thomson Speedtouch as a bridge connected to my Xbox360

I've been able to set up the Siemens so that it automatically connects to the internet when turned on.
I have disabled security on the routers to enable connection without this added pitfall (but I think I understand the security setup)
I get confused at all the different methods I've read about for getting them to talk to each other
I've also tried :-
1.
- using the same SSID and channel number for each router
- configuring the IP addresses to appear in the same range
- turning on WDS
2.
- using same SSID with different channel number
3.
- Using different SSID with different channel number (?)
4.
- using different SSID with the same channel number
5.
- entering the IP and Mac addresses as Static IP routes

but really i'm just shooting in the dark to see what happens and I've had no success so far.
I credit myself with the ability to read and understand as much as I can but I don't have the background in this field and I've come unstuck.
If I hardwire all the devices together they see each other but this is not a possible solution for anything other than testing
My questions is :
- Is it possible to connect wirelessly these devices so My computer (Netgear) and my Xbox (Thomson) can access the internet (it'd be cool if they could connect to each other but not necessary) through the Siemens connected to the broadband phoneline

Many thanks in advance for any help.