Used railroad ties, they're cheap and will last forever. I used 10" timberlok screws to tie the courses together and on the bottom 2 courses I used a 3/4 thick wall heavily galvanized pipe to secure them into the ground. To make the hole to drive the pipe I used a 7/8 spade bit with a 12" hex extension in my impact driver.
For a 3' high wall you're going to use what's known as a dead man system to keep it from leaning too much. I did a 2' wall without one and it's really leaning more than I like.
You'll also want to consider drainage, I used fist sized rocks (called 8's and 10's locally but this is regional) in the base but still put in a 5" perforated pipe to act as a french drain against the wall.