The standard 190s are indeed 5-passenger cars. The exceptions are the Sportline and the 2.3-16, which have the quad buckets.
I don't think you can simply add a seatbelt to the twin rear buckets. At least on mine, the bolsters are too aggressive to let anyone sit there in the first place. You would most likely have to install a standard rear bench seat in place of yours to allow for reasonably comfortable seating for 3.
1988 190E 2.0 Sport Euro
With Cloth/Leather sport seats, quad buckets.