Your internet bandwidth of 20Mbps will be shared by all of the computers connected to the router, but there will only be a slowdown in performance if two or more of the computers are *actively* transferring data from/to the internet. Simply having them connected to the router won't (shouldn't!) affect performance.
As far as the LAN side of the router is concerned, each "wired" computer will transfer data at 100Mbps, but "wireless" computers *SHARE* the bandwidth (54Mbps for 802.11g).
What speed do you get with the Laptop if you connect it to the router by ethernet cable instead of "wirelessly"? The reduction from 19/20Mbps to 12Mbps seems a lot, unless you are using a USB wireless adapter in a USB 1.1 port, which only supports 12Mbps.