@ophaq
Don't be in such a rush for scanlations.
The LN translations won't even finish Part 2 until at least May 2020 so starting releases before then would be confusing for almost everybody.
@Wolvenworks
The publisher decided they wanted to release both Part 2 and Part 3 simultaneously which means two artists are required as else wise you would be asking one artist to do two monthly manga which might be too much for them.
As to why they decided to release Part 2 and 3 at the same time we can only speculate that it's because the series is so long and they didn't when to wait 4 years to start Part 3 and then another 5 years on top of that to finish it.
@Wolvenworks Two reason. One, Suzuka is already drawing Part 2 and asking her to undertake Part 3 too would be too stressful for monthly release. Two, there's a vast change of environment and character design in Part 3, so a new artist could reflect that better.
MAL link:
https://myanimelist.net/manga/122851/Honzuki_no_Gekokujou__Shisho_ni_Naru_Tame_ni_wa_Shudan_wo_Erandeiraremasen_Dai_3-bu_-_Ryouchi_ni_Hon_o_Hirogeyou
@fhadielp
You should have read the comments first but to answer you the art-style is different because Ryo Namino does the art for Part 3 while Suzuka did the art for Part 1/2, and in the cover picture MC is
eight years old while at the start of Part 2 she is seven years old.
@alexsdu
There are no English translations and I don't think anybody has spoken about scanlating it either.
Furthermore, if somebody were to scanlate it before June or July of this year it'd be near inaccessible to people because it requires for you to have read the entirety of Part 2, however the Part 2 manga will not be fully released in Japan for at least another four years (let alone English scanlation) so people will need to have read the WN or LN version of Part 2. The English eBook for the final LN volume of Part 2 should come out in either late June or early July. Once the English LN version of Part 2 is completed we may have a fan-scanlation group decide to work on the Part 3 manga (albeit the audience will be small unless we get another anime season announced) or perhaps the official English publisher will start working on it (although I doubt it because with its small audience profits will likely be slim).
TL;DR No Part 3 manga English scanlations exists, just forgot it even exists.