what do i need to establish a wireless network beetween 3 computers that a bit far one from another.
The first two are close, neighbour's house, but third is about 300m away.
do I need a router, antena or something like that, I don't know much about this.

You would either need one or more additional accesspoints to repeat the signal that distance or one monster of an antenna to broadcast the signal that far. If there will be objects in the way (walls, etc) doing the repeater method might yield better results. Unless the computer at the 300m mark could have a directional dish connected to it to capture the signal better. If it is a desktop computer that isn't going to move much that would be a good option. If it is a laptop you will probably want to do the repeater method to retain some mobility.

There are quite a lot of objects on the way, 3 houses at least, small walls,
and the house no3 is below other houses, none optical visibility. desktop pc.
where should i place acesspoint? one or more?