Can anybody tell me what's the default connection pooling mechanism in hibernate 3. Is c3p0 used for default connection pooling in hibernate? if not then please tell me in detail.

The default connection pooling mechanism available with hibernate is c3p0.
Hibernate ships with c3p0, so it is to configure by adding few configuration properties to your configuration file (hibernate.cfg.xml or hibernate.properties).

A simple hibernate c3p0 configuration is shown below:
<property name="c3p0.acquire_increment">1</property>
<property name="c3p0.idle_test_period">100</property> <!-- seconds -->
<property name="c3p0.max_size">100</property>
<property name="c3p0.max_statements">0</property>
<property name="c3p0.min_size">10</property>
<property name="c3p0.timeout">100</property> <!-- seconds -->

The other connection pooling mechanism that hibernate supports are:

Apache DBCP: http://jakarta.apache.org/commons/dbcp/

Proxool: http://proxool.sourceforge.net/