The ministry said the three died after being wounded during a routine escort operation in southern Helmand province.
The fourth casualty, a Royal Marine, died at a military hospital in Birmingham of wounds sustained Monday while on patrol near Sangin, also in Helmand province.
Their deaths bring the number of British soldiers killed in Afghanistan since the U.S.-led invasion in 2001 to 149.
On Tuesday, 28 insurgents and two Afghan soldiers were killed in two separate operations in Helmand and Uruzgan provinces.
Government officials say the two soldiers and 18 militants were killed during a poppy eradication effort in Helmand province.
Some information for this report was provided by AFP, AP and Reuters.
The rest of the militants were killed by U.S.-led coalition and Afghan troops in an airstrike in central Uruzgan province.
Military officials say Afghan and coalition troops were on routine patrol when militants attacked them with small-arms fire, mortar rounds and rocket-propelled grenades.