These three Harcos are indulging in a bit of communal grooming. At the end of their shift, various groups will go soak for 30 minutes or so in the underground hot springs that are always an essential feature of any Daraz city. Harcos in particular are responsible for a lot of the hard labor of keeping the city functioning, so the soak helps keep those muscles loose and free of pain.
After the skin has softened up a bit, the Harcos exfoliate one another using the sharp rasps on their tongues. This is especially important for areas where keratin armor plates connect - if dead skin is allowed to accumulate it can cause infections. Since there's a lot of surface area to cover, grooming works best when multiple people are at it at once.
This communal grooming is good for more than just keeping clean. It's very relaxing and releases endorphins that calm the Harcos and help them sleep. And since they typically elect to groom their quadmates first, it also acts as a social cohesion activity. You rasp my back, etc.
And yeah, it LOOKS really sexy from a human point of view. To some extent it serves a lot of the same non-procreative function as sex does for humans. But, as they lack reproductive systems, Harcos are strictly asexual and don't really quite understand what humans are always going on about.