Part 3 of the EN focal spot measurement standard uses a similar method as the pinhole evaluation method in part 2. But instead of a round pinhole a slit is taken as aparatus and two separate measurements have to be done.

1 - focal spot
2 - direction of the beam
3 - maximal deviation of the symmetrical axis of the slit
The larger of both values has to be taken as focal spot value. The user of this standard has to prove, that the deviation to part 1 is smaller than +/-10%.
The slit has an opening size of 10µm with +/-5µm tolerance. Similar to part 2 there is a table for a minimum magnification - but this table starts at 100µm ...

As far as I know this part of the standard is not very often used. Also I did not found an updated version which would allow a digital image capture and evaluation like part 2 got in the year 2008.