are those fashion patterns?
here's another way that yields accurate results
make a copy of the segment you want to add the 3" notch to (or just make a cut the 3" starting anchor then re join when done)
I made a copy of it and added an arrowhead just to note direction and where the segment ends
We're going to use an Art brush, so create a 3.25" horizontal line and add a vertical notch 3" from from the start.
Select both the 3.25" line and the vertical notch and created a new Art Brush. On the Scale Options, choose Stretch Between Guides and enter 3" Start and 3.25" end. Click OK.
Select your segment and apply the newly created brush
Go to Object->Expand Appearance Menu
Notice the path got expanded into 3 objects, your original segment, the notch and the arrowhead
the Notch is created at the 3" mark and an anchor point was added to the segment. I just cut it with the Scissors Tool to measure it.
