Maid-san wa Hitozuma ~Miyako-san no Yuuwaku Nikki~

Main character's parents have died in an accident leaving him in custody of maid Miyako. He has just graduated from university, so decides to continue his father's career in writing. One day uncle's maid Saeko shows up demanding to determine which of girls is better as a maid. They decide to compete in sex techniques, and protagonist is chosen to judge their skills. The third maid rival interrupts the race and demands equal opportunity for her to display her prowess.

[From vndbreview]