Since you asked if that was the case around the time they were written, I don't think so.

Now, the only information I could find on the subject (there is not much research on the reception history of these novels within Japan to begin with, but unsurprisingly, literary analysis comes a dime a dozen) notes that the Romance of the Three Kingdoms wasn't translated and published in Japanese before the 17th century (published 1689-1694, to be exact). However, it is not known how or when exactly the original Chinese work itself managed to get to Japan. Inoue notes that happened likely around the year 1600 through Nagasaki: the oldest proof we have, according to him, that someone in Japan possessed (and read) a copy is from 1604, where it is being mentioned by the intellectual Hayashi Razan in a reading list of books he owned. [Note: this is interesting, since it doesn't match up with the information contained within a different book, see the post below]

As for the Journey to the West, its likewise been circulated during the Edo-period, although only in abbreviated versions; astoundingly, a full translation of the entire work to Japanese hasn't been published until quite recently (1960).

Incidentally, there seems to have been somewhat of a "boom" regarding such translations of Chinese historical novels during the Edo-period following the publishing of Romance of the Three Kingdoms in Japanese, with other Chinese classic novels being variously translated subsequently over the course of the 18th century onwards.

For example, the Water Margin (Suikoden) also was first partially translated in 1728 (with a complete translation apparently following a few decades later, in the 1750s) and did find a heavily inspired imitator in the novel Honchō Suikoden (1773) by Takebe Ayatori. The writer Tsuga Teishō (1718?-1794?) also was heavily influenced by the Water Margin, with his own novels Shigeshigeyawa and Hitsujigusa allegedly borrowing quite liberally. The novel also was adapted in the form of picture books (ehon) and so on.

Takebe's novel, in return, can be seen as a prototype or inspiration for the very popular "supernatural" (kaii) history novels of the late Edo-period, such as the Nansō Satomi hakkenden.

Thus, Chinese novels definitely had an influence (and if we accept the suggestion that they led to the genre of supernatural fiction, a quite far-reaching one) once they were made available to a broader audience within Edo literary circles, but that's mostly centuries after their original Chinese versions were produced.

​

References:

Inoue Taizan. "Nihonjin to 'Sangokushi engi': Edo jidai o chūshin to shite." Kansai daigaku Chūgoku bungaku gakkai kiyō 29 (March 2008), pg. 19-38, esp. pg. 22-23.

Liu Feifei. "Tsuga Teishō yomihon ni okeru 'Suikoden' no juyō." Kinsei bungei 105 (2017), pg. 31-44.

While waiting for an answer this post by u/lordtiandao gives some answers how chinese literature effected japanese literature and courts.