Ecosystem is a community of organisms interacting with each other and their environment.

population is a summation of all the organisms of the same group or species,which live in a particular geographical area,and have the capability of interbreeding.

community is a social unit of any size that shares common values,or that is situated in a particular geographical area[village or town]
(1). Ecosystem :

Ecosystem is a system of living things interact with each other and with the physical world.

(2). Population : A group of the same species that lives in one area is called Population.

(3). Community :
A Community is a set of people living in a particular space or region and usually linked by common interest.