Some problems are more common than others. The most common ones are listed in the PHP3 FAQ, found at http://www.php.net/FAQ.php3
If you think you have found a bug in PHP3, please report it. The PHP3 developers probably don't know about it, and unless you report it, chances are it won't be fixed. A form for reporting bugs is available on the PHP3 network of sites, the main form is at http://ca.php.net/bugs.php3.
If you are still stuck, someone on the PHP3 mailing list may be able to help you. You should check out the archive first, in case someone already answered someone else who had the same problem as you. The archive is available at http://www.tryc.on.ca/php3.html. To subscribe to the PHP3 mailing list, send an empty mail to php3-subscribe@lists.php.net. The mailing list address is php3@lists.php.net.
If you want to get help on the mailing list, please try to be precise and give the necessary details about your environment (which operating system, what PHP version, what web server, if you are running PHP as CGI or a server module, etc.), and preferably enough code to make others able to reproduce and test your problem.