why is IP all the same?

hello people, i would please love to hear your opinion on this.

i have 3 available public IPs on a dedicated server. i installed a proxy there and made it listen to those IPs.

for example:

http_port 192.168.1.2:3128 192.168.1.3:3129 192.168.1.4:3130

then i proceeded to test this, setting a browser to use proxy on the 3 IPs. going to ipchicken.com or whatismyip.com, i expected to get the 3 distinct IPs to show. instead, these services give me an IP which is the IP of the server, the one really bound to the ethernet device not just as a virtaul interface. and the IP that is showing is not among the 3 i use.

is this normal behavior if you use virtual interfaces? or am i missing something in the proxy configuration?