A 3G SIM card works in a 2G phone if the phone is compatible with the bands offered by the carrier. SIM cards are backwards-compatible.
Some carriers maintain bands compatible with older 2G devices and some do not. Of the major carriers in the United States, T-Mobile advertises that phones unable to use their 3G bands default to 2G bands. A 2G phone works with a T-Mobile 3G SIM card. AT&T, the competing GSM carrier, does not maintain a 2G-compatible band and advertises that its service requires a 3G-capable device. A 2G phone is not compatible with an AT&T 3G SIM card.