To be clear, doing this at all is likely to result in bad shifting for a lot of reasons, even if it works. The chainring shift assist features and intended chain contact spots on the front derailleur won't be located correctly, and that will be true at the same time that the chain is being asked to go a bigger distance than FDs have ever typically been able to handle moving them all at once. It will be at its worst with an indexed shifter, and with friction you may gain a little from being able to finesse the shift.
When you exceed a double FD capacity like this, the first consequence will be that at some point while on the small ring and shifting in back in the smaller cog direction, the chain will start dragging on the tail end of the cage where it passes over. You can't just ignore this; if you use that gear, you'll grind through the cage. Without testing, predicting which is the first combination to do this is impractical, but from what I've seen the published limit on Shimano FDs is "hard" enough to say that there will very likely be a point where it happens. Since these gear combinations are getting into the realm of cross-gears, how much it matters that they won't be usable is up to you.
Doing it with a triple FD will tend to result in the problem that when the FD height is set correctly to clear the big ring with minimal gap, the chain contact area of the inner cage won't be where you want it to be when you it to be there to shift from small to large. In other words, because you're only using about half of the FD's intended lateral movement, there's not enough vertical movement happening fast enough. I've seen this kind of experiment go wrong for this reason but don't know what exceptions might be out there to get around it.
Using a mountain/hybrid type triple FD like the Tourney linked to with an intended large chainring of 42 will never work well with a road-size ring like a 50t+. It also won't index correctly with any Shimano road left shifter.
