What are the benefits of 3 knives rather than 2 - the answer eludes me. Using the DeWalt DW733 to illustrate - it has a feed rate of 10m a minute. No load speed is 10,000 RPM - in use the actual speed will be somewhat less depending on depth of cut and wood.
Based on this there are 1,000 RPM per metre. With two blades the wood is being cut at 0.5mm intervals. In theory this will leave ripples in the finish as the knives "scoop out" the wood - the size will depend on the diameter of the block.
Simple geometry (which is all I am capable of) suggests this will be very very small. Using Pythagoras gives an answer of around 0.005mm - irrelevant.
So back to my initial question - 3 knives over 2 will make little measurable difference to the quality of the finish assuming the feed rate is the same. Sharpness of the knives is likely to be far more important.
The diameter of the block may also have an impact on cut quality - larger diameter means the knife speed is faster for a given RPM, the "ripples" will be smaller.
It does allow an increase in feed rate without compromising the quality of finish - useful in an industrial environment but a lunchbox thicknesser is more DIY than industrial of kit (normally).
Possibly of greater importance will be machine set up to ensure infeed and outfeed trays are properly adjusted (minimise snipe), feed rollers clean, extraction working properly etc.