Sending over a number from Arduino to Attiny85
As Look Alterno stated it would be easier to directly use an LCD display. But if you really want to use the 7 segment display and the ATTiny85 you can use I2C to send the number to the ATTiny. There are libraries, that can use the USI hardware ot emulate a (nearly) full I2C bus. For example my TinyWire library, that I composed from the previous TinyWireM and TinyWireS libraries. I2C needs only 2 pins (pin 0 as SDA and pin 2 as SCL I think), so you still have your 3 pins left for the display.

Then you would make the Arduino master and the ATTiny slave. Use the examples, that come with the library to learn it's usage. Note, that currently you can use the write() function only with one byte, so if you need bigger datatypes you need to split them to byte-level on your own.