My standard placement for the 3-rail shot is with the object ball centered between the two side pockets (on the center spot) and the cue ball about 10 inches straight towards a side pocket. The shot is a half-ball hit on the object ball with no spin whatsoever on the cue ball. That works fine and it is easy for a beginner to do on his own, as long as he can hit the ball hard enough.
The shot does not work so well on new cloth because you tend to get slippage rather than reversing side spin on the slippery rails.