I think the more scientific way to approach it would be to run them all separately. That way, you'd be able to taste them all individually, judge their potential contribution, and blend as desired. Then again, some people like to live on the wild side and field blend. I don't think the grapes care, so it's really a matter of how you want to approach it. If it were hybrids - new grapes, new formulas, new flavors, I'd be more inclined to do them separately and blend later, just because doing so would help control some of the unknown factors. But your 3 grapes are very frequent bedfellows, especially in Bordeaux, so you know they'll play nicely together, which supports the case for field blending. On the other hand, I remember reading somewhere that grapes that are high in flavor/color but low in tannin can be stabilized by co-fermenting with another grape that is higher in tannins. (Idea being that tannins from Grape B help polymerize the anthocyanins in Grape A). Which means that a field blend can sometimes be more than the sum of its parts.
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