1) how can I start a conference between the two calls?

Just call the conference('') without any input parameter. This will create a conference from the exiting calls.

2) if I make a third call, how do I add it to the conference that I created?

conference('thirdnumber')

3) how can I do to destroy the conference and put the two/three calls in hold?

there is no api to destroy the conference, however you can disconnect individual parties

setline(linenumber); //linenumber is the channel you wish to disconnect
hangup();

The same with hold:

setline(linenumber); //linenumber is the channel you wish to put on hold
hold(true);

any news?