Try the following:
$str = "text1<br/>text2<br/>text3";
print_r(preg_split("/<br\/>/", $str));
I'm assuming missing the closing quote "
at the end of the $str = "text1<br/>text2<br/>text3"
is just a typo.
Take a look at this page on how to specify the string $pattern
parameter: http://php.net/manual/en/function.preg-split.php