In the current episode of Dragon Ball Daima, Vegeta showcases a new form that fans have never seen him officially exhibit before. However, indirectly, this form has been seen once before – even by adult Vegeta.
What kind of form is this? Vegeta transforms into the triple Super Saiyan in the 12th episode of Dragon Ball Daima. He fights against the Tamagami from the second demon world and has to give it his all.
You can see what he looks like in the following image:

In the triple Super Saiyan form, the hair of the Saiyans grows rapidly compared to SSJ2, while they lose their eyebrows. This visually indicates that Vegeta is using SSJ3. This has never happened in the anime before, probably, because this Saiyan form costs so much energy.
But finally, Vegeta manages to use the triple Super Saiyan form. Unfortunately, the Z-crew around Son-Goku has shrunk, so we only see this special form of Vegeta as a child. However, there has already been an appearance of Vegeta transforming into SSJ3 as an adult.
The trailer for Dragon Ball Daima clearly shows the shrinking problem:
Vegeta was already seen as an adult with SSJ3
Where was Vegeta seen as SSJ3? Since the original mangaka Akira Toriyama passed away, his assistant Toyotarou takes over and will continue the Dragon Ball Super manga. He is a suitable candidate because he has already gained experience drawing the characters through his own manga called Dragon Ball Heroes: Victory Mission.
And it is in this manga that Vegeta was seen. In chapters 21 and 22, Majin Vegeta reached the state of SSJ3. He teamed up with Spopovich and Yamu to defeat his own version in SSJ4 form.
You can see his appearance in a tweet from DBZcom:

This form has even longer hair than Vegeta does in his child version. So, fans got an impression of what Vegeta looks like as SSJ3 10 years ago.
What kind of manga is this? Dragon Ball Heroes: Victory Mission is a spin-off of the Dragon Ball manga and tells the story of a boy named Beat. He lands in an arcade game of Dragon Ball, where many characters from the anime appear.
Here, the wildest combinations and forms that never existed in the official manga are possible. This also made the fight of SSJ4 Vegeta against SSJ3 Vegeta possible. At that time, the form was not 100% official, but it already showed 10 years ago that Vegeta has mastered the triple Super Saiyan and what he would look like.
However, you have to envision the black M on his forehead as absent when imagining Vegeta in his normal state as a triple Super Saiyan. Where he got the M from and what it means is explained by MeinMMO in a separate article: Dragon Ball: This is the meaning of the M on Vegeta’s forehead