As far as I know, neither the SPD nor Greens have made explicit statements against a coalition with the Left. Instead they merely opted against endorsement of such a coalition which is not the same as ruling it out. From the side of the Left, in their paper for possible coalition talks after the election they have not included their usual call for Germany to leave the NATO, so that's kind of the same thing going on. All three parties are remaining purposefully vague because they all know of the paranoia and don't want to lose votes on the final stretch of the election campaigns.
If you've watched any of the three-way discussions on TV in recent weeks, it has been pretty obvious that SPD and Greens see it as a high priority to keep the CDU/CSU out of government. Scholz and Baerbock repeatedly teamed up against Laschet despite not agreeing on everything with each other. That's why I see neither the SPD or Greens pursuing a coalition with the CDU/CSU, so all other possible options will be evaluated first, the obvious exception being the AfD which nobody likes.
Right now, red-green-red has about 50% of the current polling, so that's a majority after the insignificant parties get removed from the equation. In case that the Left fails to meet the 5% threshold, we'd be looking at ~10% of the total amount of votes getting discarded, meaning that SPD and Greens would need only ~45% of all votes to hold a majority of seats. If the Left falls below the threshold of 5%, it's likely that those votes would be lost to the SPD and that would result in ~45% for SPD and Greens combined.
As for parties abandoning parts of their agendas in coalitions, that's the necessary compromise to get something done instead of nothing. How much a party can get through in a coalition strongly depends on their share of votes in the election. The only Bundesland where you can really criticize the Greens is Baden-Württemberg, because it's the only one where they lead the coalition and could definitely get more done than they do. The reason why you see less bending from the CDU in coalitions is because in almost all of them they are the leading party.
Which is why I'd prefer a red-green-yellow coalition over red-green-red.
Not that I'm a fan of the FDP but at least there would be one somewhat conservative party to oppose the more radical left ideas. Also red-green-red would ignore all conservative voters (about 50%) and thus divide society even further.