stan - or post, post, post, etc. then fit the arris rails by following the advice I gave to an earlier poster ...
"you bend them like a longbow ... fit one end, move along the rail 2/3rds the way, one arm hook over the rail the other arm hold the rail near the end, bend the rail, guide the rail to the mortice hole with the holding hand, let go all, the rail will 'spring' into the hole. Method has worked for me for years. Tip ... do a dry run to check that the rail tenons fit into the mortices before you do the Robin Hood impersonation. The above is easier to do with 2 people."
So Stan, you now have a choice: post, arris, post, arris
or
post, post, post, arris, arris, arris.
IMO one advantage to post, post, post method is that you allow the bedding concrete to set before 'loading' with arrises, although both methods are acceptable.