I wrote this elsewhere a couple of days ago. It refers to easyphp1.8 http://www.easyphp.org
Easyphp has a www folder.
Whatever you put in there acts like your servers public_html folder.
So, put phpBB2 and phpBB3 into www.
Go to the big e icon and choose configuration>phpmyadmin
Make a database for phpBB2 and run the 2x installation (click the e and choose localweb and then phpBB2). By default the database username is root and there's no password.
Now you have phpBB2 installed you need to get your database for that and restore it using bigdump (just google it - very easy to use).
You will now have your 2x board running on your PC.
Now make a database for phpBB3 and run that installation for that.
Now choose the converter option and it will convert your 2x database.
Backup the new 3x database and restore it to the real server.