Precision amplitude 600kHz square wave generator
Maybe a fast gate driver IC driving push-pull MOS n/p channel discrete transistors powered by the 3V rail. You need &lt; 100 milliohm output resistance to get less change than 10mV with a 100mA load.
Note: It won’t take much inductance at 600kHz to ruin that low resistance. About 30nH, about the inductance of 1” of wire,
So I’m not sure how practical it is to achieve that goal.
It’s also a bit high frequency for a 555, though some CMOS versions can do it.