It is pointless to compare any REFLECTED light meter with any INCIDENT light meter, if the reflected meters are not being pointed at 18% grey card (we will dismiss debates about the better suitability of 12% grey target, for this discussion).
It is perfectly suitable to compare two meters, both reflected light meters, simply by pointing both of them at the same tonality surface, even one which is not 18% (or 12%) tonality...a featureless wall would be an example. After all, the point is merely to determine if the unknown meter matches the known meter. If you compare three reflected meters made by different manufacturers, you are likely to run into the fact that there is NO SINGLE ABSOLUTE STANDARD that all manufacturers follow for meter calibration...the ISO standard equation allows manufacturers to choose the actual value of a 'constant' in the equation! But they nevertheless should be within 0.3-0.5EV of one another