Three main venous trunks flow into the sinus venosus.
"Text Book of Biology, Part 1: Vertebrata" by H. G. Wells
The whole sinus venosus has become part of the right atrium.
"Encyclopaedia Britannica, 11th Edition, Volume 3, Slice 7" by Various
The sinus venosus receives the blood from the great veins (ductus Cuvieri and hepatic veins).
"Encyclopaedia Britannica, 11th Edition, Volume 14, Slice 3" by Various