Not knowing much about emulsion making yet, I'm surprised to hear that 3-separate emulsions are mixed. I guess I thought (somewhat naively) that the sensitizing nature would be applied equally to each grain. That is, a single theoretical grain would become sensitive to the full spectrum, as opposed to 3 grains from separate emulsions each "taking care of" a portion of the spectrum.
There are a variety of methods to achieve pan sensitization. Bill described a method I posted quite a while back for making Pan Matrix Film, but you can also get a panchromatic dye to do the job. Placing 3 dyes on one grain (or even 2) is rather difficult though.
Can you give some examples of panchromatic dyes ?
I actualy tried,with one emulsion,adding both SDA3057(at 150mg/M Ag) and SDE3008(at 50mg/M Ag). It looked pritty good, as judged by a shot of the Kodak color chart. I went back to splitting batches though. Just to be on the safe side. Also, when using a split beam camera, I can actualy use a differntly sensitized emulsion behinde each of the color filters. I like that for several reasons.
When you add 2 dyes to 1 emulsion, the more tightly adsorbed one can kick the other dye off the grain and you lose sensitivity in one region of the spectrum. That is why I suggest mixing 2 differently sensitized emulsions.
As for a pan dye, well that is the problem. I have not yet found one commercially. We had them at EK. There is a problem with these too. Their spectral distribution may not match the spectrum you wish to capture, and it may vary from emulsion to emulsion. That is another reason I prefer the blending method. Of course this is only a problem with B&W materials. I've been fortunate that all of my work with spectral sensitization was in color where it was mandatory that a minimum of 3 emulsions be used. The B&W work was purely "theoretical" in meetings and such reviewing the work of the B&W people.