The Gecko G-540 comes with 4 drivers built in. Yes, this is more than you absolutely need, but the package is so convenient, with the breakout board integral, that it's worth getting anyway. It's also handy to have a spare drive - on a 3-axis machine, you can use two drives to power slaved motors on the X axis, or have one spare in case a drive goes out on you. Or you might want to add a rotary 4th axis at some point.