Does iphone 3G data disable when I connect to Wi-Fi?

Does 3G disable if I connect to Wi-Fi or do I need to disable 3G manually.

It doesn't disable, but if you are connected to Wi-Fi it will automatically choose Wi-Fi over data. The only way for you to have data and not Wi-Fi would to do the reverse and turn off Wi-Fi

No the Wi-Fi takes priority over the data.

No you have to do it manually

Yes. Wi-Fi is priority where available. When it is no longer available it will switch back to 3G.