PCF8574 I2C Address Selector
The datasheet is rather vague on the subject, but it appears that the A0-A2 lines have internal pullups.

If this is really true, then connect your switches between the address lines and ground.  A closed switch forces the line low, and a open switch allows it to float high due to the internal pullup.

You might want to send email to your local TI field application engineer and ask for clarification on whether these inputs really have internal pullups or not.  The wording around that in the datasheet is quite poor, and it wouldn't hurt for the FAE to bring this back to the factory to have it fixed.