hi.
there are some sites that gives ability to add the RSS and site maps and they are not depends on our web site's system.
now i want to know it is good to use them in our phpbb3 forum?

Yes, good, but not really handy since you need to manually update them.

The only thing to make sure of when using such system is to avoid bad url and duplicate listing.
Usually, these tools are able to follow the robots.txt directive, which is enough for a phpBB3 forum optimized by phpBB SEO (if of course you implemented the robots.txt).

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tnx,sorry can u plz explain a little about robot.txt ?