Is it really _as weird_ as calling SF Saint Francis? This link would suggest that the name "Three Rivers" exists at least in some capacity
http://www.cqsb.qc.ca/MyScriptorWeb/scripto.asp?resultat=253...
And the Wiki article says
>Traditionally, Trois-Rivières was referred to in English as Three Rivers
although it then goes on to say that this is becoming less and less common.
Edit: and a completely irrelevant but somewhat interesting piece of information, the city's inhabitants are called Trifluvians. I say interesting because the fluv- part obviously comes from the other French word for river, fleuve, rather than the one used in the actual name of the city, rivièr.