As much as I love RV, I admit that this is exactly what I'm feeling too. SM needs to promote RV the right way. I don't want RV to be the next f(x). It still hurts while thinking about them (I hope they come back).

Firstly, there's kind of a false analogy going on here, with regards to the words 'big three'. Big Three refers to the companies, not the artists. There was never a stable Big Three ranking for girl groups, since it fluctuated a lot, depending on the year.

Secondly, I think it's important to carefully define what you mean when you think of 'success' or 'status'. People take different factors into account when defining those two things - usually they would pick the ones that favour the groups they stan. So it's a bit hard to say who's "rising" or "falling" in terms of status, because it all depends on how you calculate 'status'. Here are a few factors that people may consider, but may place different weightage on:

- Music Sales:

--- Physical

--- Digital

--- Streaming

- Brand Reputation:

--- Merchandise sales

--- Advertising CFs & Sponsorships

--- Broadcast appearances

--- Ability to make the news (usually in terms of records broken)

- Social/Cultural Impact:

--- Social media influence

--- Opening new markets for Kpop
--- Celebrity partnerships / networking

--- Influence outside pop culture (e.g. helping S. Korea build diplomatic relations with other countries)

- Fanbase:

--- Size of fandom

--- Concerts / tours ticket sales

--- Fan event attendance / profitability

I'm not part of the Red Velvet fandom, so I don't really know for sure how well they're doing in all of these factors, but as far as I know, among 3rd-gen girl groups, their album sales are only second behind TWICE. [Source] And they did have quite a bit of cultural impact when they performed in North Korea for Kim Jong Un.