I have an iPhone 3 that can't connect to the wi-fi?

I have a computer, iPod touch gen 4, two iPhones, ect. That use the Wi-Fi so I know that it's not the Wi-Fi. What I want to know is: Can I fix my iPhone's problem? Also how expensive will it be?

Check Wi-Fi antenna

Enter an IP address manually like 192.168.0.77 and see if connects if it does there's an address conflict on the DHCP page of the router

There are different Wi-Fi modes: a/b/g/n and 2 different frequency bands. Your old iPhone likely uses only the "a" mode. If your Wi-Fi wireless router doesn't offer that old mode, your iPhone won't work. If that is the case, the solution is either a newer iPhone or a new router that supports the old "a" mode.

Tap Settings > Wi-Fi and make sure Wi-Fi is turned on. Restart your iOS device. Still can't connect? Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. For safe and secure use of your smart phone, you also need Leo Privacy Guard app, Just a click to lock all apps.