In main.c, edit the php3_module_startup function and add the appropriate cfg_get_string() or cfg_get_long() call.
Add the directive, restrictions and a comment to the php3_commands structure in mod_php3.c. Note the restrictions part. RSRC_CONF are directives that can only be present in the actual Apache .conf files. Any OR_OPTIONS directives can be present anywhere, include normal .htaccess files.