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OK, the only magic to the "3+3" rule is that if you plug those numbers into the horribly unwieldy formula for Fisher's exact method of calculating probability, you will get a probability (p) of 1/20, or a 1 in 20 chance that the reactions could have occured by chance, or a 95% confidence level that your conclusion is correct. (This is a totally arbitrary number by the way.) Lower p values (1/15, 1/9 etc) allow for too much chance of random association. Higher values (1/28, 1/56) show that there's a much smaller chance that your conclusion is incorrect. But there are other magic combinations that will give you an acceptable p: 5 and 2 (1/21), 4 and 3 (1/35), 6 and 2 (1/28) and so on. You do not necessarily need 3+3. See Goodchild's reference. You run into problems when you only have one positive or negative cell: 7 and 1 (p of 1/8), 8 and 1 (1/9), etc. You would have to get to 19 and 1 to get the magic p of 1/20. If you think about it non-arithmatically, what if your one reactive panel cell is also positive for an unlisted low frequency antigen? What if your one negative didn't have serum added or isn't reacting for some other technical reason? So you don't necessarily need 3 Cw+ cells; 5 or more neg and 2 pos would suffice. And you don't need 3 Js(b-) cells; 19 pos and 1 neg would be OK statistically. The problem I see with the high incidence antigens like this would be that with only one negative cell with which to rule out, you would still have a bunch of other antibody choices you would like to rule out, hence the need to test more negative cells. So, pedantry aside, the bottom line is "don't base your ID just on the reaction with one cell". A second cell of similar makeup coupled with the pos or negs from the rest of the panel should bump your p past 1/20.

AABB changed the standard a few years ago to 2 and 2 to accommodate reference labs - due to low/high incidence ags and the scarcity of either positive or negative cells. I try to use 3 and 3. I can r/o Kell and Lu system ags with cells heterozygous for ag expression (unless I enzyme pretreat, then I will r/o with heterozygous cells for those ags not sensitive to enzyme - Rh, Kidd).