Weather will likely play an important role when you plan your trip to Três Rios, but bear in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually February and January, with an average temperature of 23°C, while the coldest months are July and August, with an average of 18°C. The rainiest months in Três Rios are December, January, November and February, with each month seeing an average of 327 mm of rainfall.