I’m glad that you are attuned to your yeast and realize that some strains are “killer factor positive” and one is “sensitive.” I really wish that the yeast industry had come up with a different term than “killer,” it makes it sound like yeast cells are going to, like some monster from a 1960’s B movie, take over the neighborhood and eat everything in its path! What it means in practice is that yeast strains with “killer” factors are more likely than others to dominate and eventually take over a fermentation if present in large enough numbers. There’s a point at which at low enough concentrations, even killer-factor (KF) yeast will be present in a given fermentation but will not be able to dominate if you inoculated with sufficient non-killer yeast at the beginning.
It’s clear that you’ve chosen your three yeast strains on purpose, however, and to make sure you only have those chosen strains impacting its assigned lot of grapes, you should make an effort to clean and sanitize your punchdown tool between punchdowns. This can be as simple as rinsing your punchdown tool with hot water, then spritzing it with a 70% ethyl alcohol solution, with a “One Step” sanitizer solution, peracetic acid solution, or similar. I would start your punchdown round with your “sensitive” yeast strain first, so that there’s less chance of one of your “killer” yeasts getting carried into it from the others.
It’s unlikely you’ll get a stuck fermentation with your non-KF yeast based on that fact alone. As long as you inoculated your fermentation at a high enough rate (about 0.25 g/L), aren’t cross-contaminating during your work, and you have a healthy must, I wouldn’t worry about it too much.
