The answer is the Aar (another accepted spelling for the Aare.)
The three-letter word for a Rhine tributary is "AAR."
The Aare is a tribuatry. Another option is Ruhr as a 4-letter tributary of the Rhine.
It does not flow into a sea. It is a tributary of the Rhine River.
The Ruhr is a 217 Km long river in western Germany and is a tributary of the Rhine. It flows into the Rhine in Duisburg.
Amongst others - Madon, Terrouin, Moselotte, Dhron.
The river Rhine has approximately 40 different tributaries. These include the Main, Mosel, Neckar, and Meuse. the Mosel is the longest tributary.
Aar
The Arve River is a tributary of the Rhone River. It begins with the letter A.
c
The Isere River in France is a tributary of the Rhone River.
Aro
Isere
Oka