The manga was about 1 guy and 3 girl

the guy and 2 girl was highschool student(they are on different prefecture and school), the other girl was just an elementary student around sixth grade i guess.

The world has gone extinct, a weird black creature like alien, absorbed people and the people then vanished. It become a huge panic, nobody can beat the black alien, it then absorb all people till only 4 person remaining.

Then the black alien vanished, the survivor was 1 guy and 3 girl, they then decided to live their live together and solve the mystery where the people who absorbed went.

I think you are looking for Chikyuu no Houkago 🙂

thankssss alot