how can i use 2 internet connections on 3 computers.?

[jivepacketrat]

You cannot share two Internet connections to a network unless you are using Link Aggregation Groups at both ends of the two links. At least one vendor has been successful at creating a LAG over two devices, and a couple of others are either doing this or will soon. The problem also is that they must be the same service provider. You have to be able to run link aggregation at your end of the Internet connections as well, I am unaware of any home device that will do this. The switches and routers that do this normally will cost in the thousands of dollars.
Your IP addresses are not Internet addresses they are private addresses that do not appear on the Internet. So the service provider device is acting as a router or gateway to their network and they will have a public IP address that is used by the Internet to get back to that device. The other Internet connection will also have a public IP address, to get back to that device. You have to be really careful in what you are attempting as you can cause loops towards the Internet, and that would not be good.
To do this properly you will need a high end device that supports LAG and two connections to a Service Provider that supports LAG, they are called other things by some vendors, like teaming, Multilink trunking, etc. Then you will connect both connections into a WAN port on your device, and all of your computers into that device as well. The devices usually have 24 ports at a mimimum.