Using the WordPress Default theme, in the wp-content/themes/default/index.php, change from this:
<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<div <?php post_class() ?> id="post-<?php the_ID(); ?>">
<h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>
<small><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></small>
<div class="entry">
<?php the_content('Read the rest of this entry »'); ?>
to this
<?php
$count=0;
?>
<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<div <?php post_class() ?> id="post-<?php the_ID(); ?>">
<h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>
<small><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></small>
<div class="entry">
<?php
$count++;
if ( $count < 3) {
the_content('Read the rest of this entry »');
} else {
the_excerpt();
}
?>