WordPress multisite and redirect to subpage

An htaccess rewrite rule ought to do it. Check out this post https://mediatemple.net/community/products/dv/204643270/using-htaccess-rewrite-rules for examples of what you might do.

The one you use will depend on whether content in zzz is the same as the bbb site. If you just want to redirect everything (any page) on bbb to zzz, then this should work:

Options +FollowSymLinks
RewriteEngine On
RewriteRule ^bbb.*$ http://example.com/zzz/ [R=301,L]

That will redirect anything on bbb to zzz (ie: http://example.com/bbb/somepage to http://example.com/zzz (home page, whichever it is).

Remember to adjust for https if necessary. Other options are in the above link.