Can my iPhone 3GS SIM card fit in any other brand phone?

Question says it all

How SIM Card Transfers Work

When you insert your SIM card into a compatible device, whether it's another AT&T phone or an unlocked one from another carrier, your phone will connect with the AT&T network and within several seconds, allow you to make phone calls and send messages, from the same phone number you were using on your iPhone. As you click through your Contacts menu, you'll notice that some, but perhaps not all, of your iPhone contacts appear, depending on whether you earlier set your iPhone to save contacts to your SIM card.

Limitations

Since SIM cards function only to transmit signals to and from cell phone carriers and store contacts' phone numbers, the most you can expect when you insert your iPhone SIM card into a different device is that you'll be able to make and receive phone calls using your old phone number. Nothing about an iPhone's SIM card is different from the SIM card in any non-smartphone, so inserting an iPhone's SIM card into a phone incapable of accessing the Internet won't make it suddenly capable of getting online.

I don't see why it wouldn't. The 3GS uses the miniSIM (which most just call the SIM) So the majority of phones will be able to accept them just fine.
What is more important though is if those other phones are unlocked or able to ID the SIM.