As usual I am late to the conversation,
and this is an old conversation, but the question is still valid and the answers good.
I would like to add that the 3-wire method is THE way to calibrate thread plug gages. Thread wires are available in inexpensive 'sets' which do not use the "best wire" sizes. If you wish most accurate measurement the best wire size should be used to take the measurement. With the best wire size the wire is sized to touch the screw flanks at the pitch diameter cylinder. Here is a link to a list of best wire sizes and their constants
. The inexpensive wires will more likely touch the flank angles at some point above or below the pitch diameter cylinder and in doing that will give a faulty measurement if the flank angles are out-of-tolerance or worn. By using the best wire size you will be using the same tool that a gage maker or calibration laboratory would use, and in doing so your measurement will more likely match the value that they would get if they measured the gage.
The gage standards give an amount of pressure to be used when measuring thread plug gages with thread wires. For a shop application with a micrometer it is unlikely that the pressure will be able to be controlled, so again there may be some discrepancy in the measurement between two people, but unless it is near the size limit of the thread gage the variation should be insignificant.
There are many styles of threads and each has their own standard. You may spend a small fortune in purchasing the official standards so that you can have the correct numbers to calibrate the gages. Once you own the standards you will spend another small fortune in reading them and doing the math required to determine the sizes and tolerances for each feature. I suggest thread engineering software to do this for you. The price of the software will be less than the price of the standards and on top of that much time will be saved by not doing the calculations. Here is a link for a Free 30-Day Trial of ThreadTech
software. There are other brands of software which will work as well as ThreadTech. It is just the only one for which I have a link for a free trial.
I hope that you find this information interesting
well at least helpful.