A lot of existing PHP 3 code should be ready to run without changes, but you should still know about the few differences and take care to test your code before switching versions in production environments.
PHP 4.0 and the integrated Zend engine have greatly inproved PHPs performance and capabilities, but great care has been taken to break as little existing code as possible. So migrating your code from PHP 3.0 to 4.0 should be much easier than migrating from PHP/FI 2.0 to PHP 3.0. A lot of existing PHP 3.0 code should be ready to run without changes, but you should still know about the few differences and take care to test your code before switching versions in production environments. The following should give you some hints about what to look for.