"I build a worker after all the tiles that yield 3 food unimproved are being worked."
If you do the math, you'll see that this is suboptimal, especially if you're counting on choprushing your first settler. Those 3 food unimproved tiles only give you one more food/hammer per turn toward your first worker (the extra citizen takes two food to support itelf)> Compare to the 5 food the tile could produce if improved.
Basicially, non-improved tiles are barely worth the extra citizen they require to work. Get those workers on the job ASAP.