So-called green-on-blue attacks have spiralled this year, with a total of 61 NATO troops killed by members of the Afghan security forces, fuelling distrust between the allies in the war against Taliban Islamist insurgents.
The French casualties prompted France to withdraw combat forces from Afghanistan earlier than planned.
Afghan army soldier Abdul Sabor was convicted in July of killing the soldiers on January 20 while they were jogging within their base in Kapisa province in eastern Afghanistan.
Sabor is the only Afghan convicted of carrying out such an attack to have been sentenced to death.
"The appeal court has confirmed the decision made by the primary court – his appeal was rejected," defence ministry spokesman Zahir Azimi told AFP.
The case will now be automatically reviewed by a higher court, and Sabor, who was 21 at the time of the attack, will have the right to appeal to President Hamid Karzai for clemency.
The decision to put France on a fast-track exit timetable sparked concern among some members of the US-led military coalition, which is not due to end its combat mission until the end of 2014.
France will have pulled 2,000 combat troops out of Afghanistan by the end of this year, leaving 1,500 soldiers behind to help with training and logistics.
NATO says about 25 percent of the insider attacks are caused by Taliban infiltrators but the rest stem from personal animosities and cultural differences between Western troops and their Afghan allies.
Efforts to tackle the issue include orders that NATO soldiers working with Afghan forces should be armed and ready to fire at all times, even within their tightly protected bases, and the issuing of cultural guidelines.
The guidelines, drawn up by the Afghan defence ministry, urge their soldiers not to take offence if NATO colleagues exit the shower naked, swear or ask to see pictures of their wives.
The 28-page brochure tells Afghan soldiers these things are normal behaviour and no reason to open fire.