How can I unlock my old iPhone 3 to change sim card for travel?

Hi, armnleg2.

Thank you for visiting Apple Support Communities.

I understand that you want to use an old iPhone with different SIM cards while traveling. Most likely the iPhone is locked to the carrier that it was originally used with. Here are the steps on how to request a device to be unlocked.

How to unlock your iPhone for use with another carrier

Cheers

Make sure when you follow the Apple instructions that you first restore the phone as NEW and not from backup. Then you should receive a confirmation that your phone is unlocked. After the confirmation you can restore your phone from backup. If you do it directly the phone is likely to not be unlocked.

Only the carrier to which it is locked can unlock it.

Contact that carrier and ask if they offer this service and if you qualify