Follow the V3 install instructions, with these few changes: 

Database

I'm assuming that you've cloned Lamu into folder c:\wamp\www\Lamu

Use localhost/phpmyadmin to create a database.  I've called mine lamutest.  Your database.php should look something like this: 

return array
(
 'default' => array
 (
 'type' => 'MySQLi',
 'connection' => array(
 'hostname' => 'localhost',
 'database' => 'lamutest',
 'username' => 'root',
 'password' => 'mypassword',
 'persistent' => FALSE,
 ),
 'table_prefix' => '',
 'charset' => 'utf8',
 'caching' => TRUE,
 'profiling' => TRUE,
 )
);

Where 'database' is the database name that you chose; 'username' is the user that you set up in phpmyadmin (look at the 'users' tab: my user is root@127.0.0.1) and 'password' is the password that you set up for that phpmyadmin user. 

Base_url

Like with Ushahidi v2.x, your baseurl is going to be slightly different to everyone else's.  

In init.php, for Ushahidi v3 installed at c:\wamp\www\Lamu, it's

 'base_url'    => '/lamu/httpdocs',

In .htaccess, it's 

RewriteBase /lamu/httpdocs/

Minion

Minion won't work if you can't run php (you can test this by trying "php -v" in your terminal window).  To make this happen in wamp,  find your system path (on windows 7, that's control panel > system and security > system, then advanced system settings, (lhs list) and add a link to your php to it: for my Wamp install, that means adding the text ";C:\wamp\bin\php\php5.3.13" to my path; you might want to look in C;\wamp\bin\php to see what the right address for you is (e.g. you might have a different version of php).

In your terminal window, go to your top-level directory (in this case, c:/wamp/www/Lamu) and run minion with this command: 

php minion --task=migrations:run --up

Go to http://localhost/lamu/httpdocs/

Smile: you've got your own copy of V3 working on Windows!