Has Ukraine responded in any way to the 2018 letter sent by three Democratic senators?

Question

Trump has raised the issue of a 2018 letter to Ukraine, sent by three US senators (all Democrats) . As CNN summarized this letter:

it expressed concern about a New York Times article that said Ukraine had -- with the aim of avoiding Trump's anger -- stopped cooperating with the Robert Mueller investigation and frozen investigations into the Ukraine-related activities of Paul Manafort, Trump's former campaign chairman.

It urged Ukraine to "reverse course" if the Times story was accurate. [...]

The letter concluded with three questions to Ukraine's prosecutor general. It asked whether his office had indeed taken steps to stop cooperating with Mueller, whether anyone acting for the Trump administration had asked Ukraine to do so, and whether the Mueller probe had been raised during previous bilateral meetings between the two countries.

Did Ukraine officials (e.g. their prosecutor general) respond in any way to the Leahy-Durbin-Menendez letter, specifically to the final 3 questions asked in there?

Answer 1

Did Ukraine officials (e.g. their prosecutor general) respond in any way to the Leahy-Durbin-Menendez letter, specifically to the final 3 questions asked in there?

No, according to Mother Jones.

Prosecutor in Trump-Ukraine Scandal Refused to Cooperate With US Congress,SEPTEMBER 30, 2019

Three Democratic senators—Patrick Leahy, Dick Durbin, and Robert Menendez—quickly fired off a letter to Yuri Lutsenko, then Ukraine’s general prosecutor, expressing their “great concern about reports that your office has taken steps to impede cooperation” with Muller’s investigation.

...

The unanswered letter sent by the senators to Lutsenko leaves a big hanging question: Did Trump in any way try to pressure the Ukraine government to impede Mueller a year before Trump phoned Zelensky and asked for this favor?

Answer 2

Has Ukraine responded in any way […] ?

Yes, "in some way".

Prosecutor General Yuriy Lutsenko has responded by making public his response to NYT journalists Andrew Kramer and Julia Mendel dated earlier, 20 Apr 2018 (scans of the letter available).
Which make a perfect sense: the Menendez-Durbin-Leahy letter has been inspired by the article by Kramer-Mendel @NYT.

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specifically to the final 3 questions asked in there?

As per the 3 factual questions:

On 3 May, in his interview to Reuters, Lutsenko stated that the FBI hasn't responded Ukraine's requests:

Asked about progress on investigations in Ukraine relating to Manafort, Trump’s former campaign chairman, Lutsenko said he was told in autumn by the Federal Bureau of Investigation (FBI) that they had launched their own investigation and could not provide information at this stage.
(highlight mine)

Also, according to Deputy Attorney General Yevhen Yenin,

The US law enforcement — neither the FBI, nor Muller — have asked the Prosecutor General's Office (GPU) for any legal assistance in connection with the Manafort case. That is, there was no cooperation to restrict at all.

On the other hand, it should be understood that in the current political situation Ukraine would not rush to irritate Trump and his administration because Ukraine depends on the US' military aid to withstand the foreign armed invasion.

Summarizing sources listed above, the three answers seem clear:

Q1: whether his office had indeed taken steps to stop cooperating with Mueller
A1: There was no cooperation between the GPU and the Mueller probe; there was nothing to stop;
the internal GPU's investigation of Manafort's affairs in Ukraine has never been (officially) stopped;

Q2: whether anyone acting for the Trump administration had asked Ukraine to do so
A2: There was nothing to stop.

Q3: whether the Mueller probe had been raised during previous bilateral meetings between the two countries
A3: the GPU is not in position to comment the information about bilateral meetings between President Trump and President Poroshenko.