The victims were a 77-year-old man and his 55-year-old wife who lived in the apartment, and a four-year-old boy believed to be a relation, Aargau police said in a statement on Friday.
The exact circumstances are yet to be determined, but according to initial findings police believe one of the deceased may have killed the two others with a gun before turning it on themselves.
A firearm was found in the apartment, said police.
Relatives of the victims called police after being unable to contact them.
Arriving at the scene just after midnight officers found the apartment locked and had to call a locksmith to get into the property.
The public prosecutor has opened an investigation and ordered autopsies to clarify the circumstances surrounding the deaths.