You're ignoring other alternatives to speak specifically about a single version of the activity, and then claiming that that version is relevant to all of them.
And obviously your statements haven't been obvious, because while I've been reading what you said, I haven't understood that you were referring to players just randomly finding groups of 3 enemies to throw themselves at with no chance at backup. How could I? You never indicated such until this post. The closest you came to it was:
There is no way someone doesn't know that diving a 1v3 will result in a death when they are already behind.
Which makes me think of the Leona or Nautilus who's behind, but engages on 3 anyway because they think the rest of the team will follow up on it and make it worth.
Now, to refer to the extremely specific situation that you are now talking about: Assuming that such activity is perpetuated throughout the game, then yes, it's almost certainly intentional feeding and therefore toxic.
But in the general sense that every prior post of your thread seems to imply, no, merely having a high number of deaths is not inherently intentionally feeding, nor is it inherently toxic. Although it does tend to provoke unnecessary toxicity.