Redmine

There is no problem at all, just setup as many name based virtual hosts (domain A" "domain B" and "domain C") as you need and point it to each redmine's instances.
This can be done with:
- nginx: create configs for each domain with same directive listen and different directive server_name and proxy_pass it to unique for each domains ruby back-end web-server.
- create as many folders with redmine's content as you need and start ruby back-end web-server for each of them.

If I do the technique as above what will separate the database can be made?
I've tried, but a database merge ... could ask her not a complete guide? I really need it ...
Pavel mas thank you very Potcheptsov above answer gives the solution for me ...

In each redmine/config folder you have database.yml.
You need to create different databases and write their names in those yml files.