Someone needs to edit this post again -- the code tags are not formatting the rewrite code properly.
As for the the htaccess file, the RewriteRule directive looks bad to me. I suspect you are trying to match any sequence of characters that are not a slash with
([/]) but that that's totally wrong because a) the
[/] matches only slashes and you don't have a wildcard character in there to match more than one.
Also, your rewrite rule only rewrites requests that end in .html which your example urls do not.
You should also consider whether any of these requests might have additional query strings due to session ids or pagination or sorting, etc.
Maybe try this RewriteRule instead:
RewriteRule ^activate/([^/]*)/([^/]*)/([^/]*) /activate.php?activation_secret=$1&customer_guid=$2&activation_guid=$3 [L]
I strongly suggest you make use of a rewrite tool like this one.