Why not just mark your trowel? Its already divided in two by the handle so it wouldn't take nearly as long as knotting string. I use mine as a rough measure.

Hi, I agree with Steragram, if you mean to use the string for planting out purposes, the 3" measure is only a rough guide, given based on the eventual spread of the plant, ie, a 3" planting distance would be based on a small plant, which would only spread about 1 1/2" before meeting up with the plant next to it, which would also have spread 1 !/2", there isn't much in gardening that has to be an exact measurement, Derek.