Second, the short answer to your question is that you do not know enough to use mod_rewrite. Rather than repeat my tutorial here, please read the REGEX section of the tutorial Article linked in my signature.
My question to you, for your RewriteRule, is where do you generate $1, $2 and $3? Moreover, you’re looking at the whole problem backassward as you MUST redirect to something Apache CAN serve which, by the looks of things, you’re not doing. Again, though, the tutorial has helped many members and I’m sure it can help you, too.
If this is a WordPress problem, then I recommend NOT messing with it as doing so will BREAK the navigation. Know that WP “reads” the original request, parses it to determine which of its modules to use then fetches them to do the work to create your page.
Look in WP’s administration section as there are options to change the URI format.
As an aside, only your .htm “link” above can actually be served by Apache but ONLY if there is such a file in existence (which, if you’re using WP, I’ll guarantee it does NOT exist).
Therefore, to understand what mod_rewrite does, I recommend you read the tutorial. If you merely want a “one liner” to solve an indeterminate problem, you’ve come to the wrong place as I’m here to help members LEARN, not to feed “script kiddies.”
Please note that I’m NOT calling you a “script kiddie,” I’m just saying that I don’t do coding for members.