"Evacuation teams are working tirelessly to rescue the injured", said Ugandan police spokeswoman Emilian Kayima, adding that all three drivers had died.
"The death toll is now at 48 including 16 children," Red Cross spokeswoman Irene Nakasiita told AFP.
The accident happened on Friday night in Kiryandongo, about 220 kilometres (140 miles) north of the capital Kampala.
The bus collided first with the tractor before hitting a truck transporting beer.
Local media said the death toll exceeded 30. Uganda has one of the worst records for road safety, due to the poor condition of vehicles and roads as well as dangerous driving.
Female crew member dies after fall from plane
More than 9,500 people died in a road accident in the country between 2015 and 2017, according to figures from the transport ministry, with the situation worsening each year.
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