Dear Anonymous A,
The answer to your question is: it really depends.
You have helpfully defined the grade bands used at Harvard Business School, so one can easily infer that a grade of 3 is bad and that a grade of 4 is very bad.
We can also infer that as Harvard Business School is one of the world's prime destinations for MBB recruiting, that the assigned campus recruiters will be intimately acquainted with this ranking system. You can therefore expect that they have some loose internal guidelines on how to screen out CVs on a grade basis.
In your case, let us assume you have taken just as many classes as the average applicant from Harvard Business School to MBB when you apply. Well, that presents a healthy distribution to review:
+ Are you scoring 3s on only a handful of your courses? If so, not so bad.
+ Does that handful line up with the normal distribution of your class at Harvard Business School, or is your entire transcript a left-tail outlier in 3s and 4s? If so, not good.
+ Are you getting 3s and 4s in flagship classes that MBB historically cares about? If so, very bad.
And, these sorts of rhetoricals go on and on.
In general, if your grade profile matches ~80% of the class (including with a few in 3s, but definitely no 4s), your CV will not be penalised at the CV review stage in a normal recruiting year.
That said, once you are post-CV review stage and you are invited in for tests/games and, hopefully, interviews, your suitability will be assessed solely on the merits of your performance and your CV will cease to be an issue. This, I can attest from experience, is the case at McKinsey. I should further add (to give you complete context), that if a CV is rated strongly due to outstanding grades and that candidate is cleared through but somehow stumbles in the first round of interviews, that candidate will also be assessed solely on the merits of their performance and will not be cleared through. There will be no consideration given to how fantastic the grades were, and that will be that. I would assume the same applied at BCG and Bain, but cannot say with certainty.
One important caveat: the selectivity in the MBB recruiting process is so high at the early stages, that one could fill quotas exclusively with people who have mostly excellent and above average grades making it all the harder for people with mostly average grades to be considered.
All the best.