Threshold decryption in multi-key homomorphic encryption
Everything you write looks correct. However, you may be expecting the distributed decryption protocol to have a security property that it does not (and was not intended to, and really cannot in your example) have.

Specifically, the Mukherjee-Wichs paper you linked defines security to say (roughly) that, given the evaluated ciphertext, its underlying plaintext, and the secret keys for all but one of the parties, we can statistically simulate that party’s partial decryption. This corresponds to all but one of the parties being semi-honest, and protecting the privacy of the remaining honest party’s input (as far as is possible).

In your example, the evaluation function just sums the three inputs. Because all but one of the parties is semi-honest, the adversary knows all their inputs. It is also entitled to the output sum of all the parties’ inputs. This means it can trivially compute the honest party’s input! So, there is actually no “security violation” arising from the fact that the decryption protocol implicitly reveals the honest party’s input, because that input is implicitly revealed by the function itself.

If you consider a different evaluation function that doesn’t implicitly reveal the honest party’s input, then I think you should see why the “smudging” noise works as intended.