Can I read from a memory card with three pins?
Not with an ATTiny85, and not with so few pins, no.

However, all is not lost.

The ATTint85 has 512 bytes of EEPROM inside it where you can store your login details. All you need is some way of telling setting that data in your sketch from the PC - and the simplest way is probably through a USB serial connection.

If you don't have a serial connection available (I think Digispark has a custom interface they use) then you can use whatever interface you have available to do the job.