What is running right now?
You have three(3) options to go about looking at what is running in MySQL
OPTION #1 : General Log
You could simply activate the general log and look for the timestamp between 3:00-3:30 AM. You could either have the general log as a text file or a MyISAM table. Here are my posts on using the general log:

How to enable MySQL general log? (Jan 08, 2012)
How to keep MySQL general_log after I restart my computer? (Dec 10, 2011)
How do I output MySQL logs to syslog? (Jun 30, 2011)

OPTION #2 : Slow Log
Same setup procedure as the General Log. Here are my past posts on this:

MySQL slow log configuration (Mar 30, 2012)
Create a custom log table in MySQL (Nov 18, 2011)

OPTION #3 : Query Digest
The problem with Options 1 and 2 is that they record only completed queries. If you are trying to catch long running queries in the act of running long between 3:00-3:30AM, even before they get recorded in the slow log or general log, you have to go with setting up a crontab.
I usually use MAATKIT's mk-query-digest (most prefer the most current pt-query-digest). Here are my posts on how to script a crontabed Query Digest:

Investigate peak in MySQL throughput (Apr 24, 2012)
MySQL general query log performance effects (Nov 24, 2011)