It is the flag of Wallonia region of Belgium, which in broadest terms are the 5 southern, French-speaking provinces of Belgium.
There is no specific country with just Red, Yellow, and Grey, but the country that closely fits that category is Uganda. It has Red, Yellow, Grey, Black, and White.
South Ossetia
China's flag is red with five yellow stars.
Lithuanian flag has yellow, green and red horizontal stripes.
Several countries have a blue, yellow, red, and white flag:KiribatiCape VerdeMalaysiaPhilippinesSerbiaSloveniaTuvaluIn addition, the Buddhist flag is blue, yellow, red, and white.
poland
A country whose flag has the colours green, white and red
Brazil has a green flag with a blue circle inside a yellow diamond.
Romania
Colombia-South America
Benin's flag is blocks of green, red, and yellow. Green recalls hope and revival, red the courage of ancestors and yellow is a reminder to preserve the country's wealth.
Flag is Red background with yellow sun. Red means bloodshed from Macedonians protecting their country throughout the centuries, Yellow Sun means "New Sun on Liberty"
French Guiana
Romania