.htaccess: param url to clean url - regex

Question

I am trying to turn an ugly url with parameters into a nice url. At the moment I have:
http://myasite.com/index.php?reg=uk&area=london&id=16
Which I would like to have like so:
http://myasite.com/uk/london/16
I have tried using this .htaccess:
RewriteEngine On
RewriteRule ^/?$/?$ index.php?reg=$1&area=london&id=$2 [L,QSA]
Which I got from an online generator however when I run the page with /uk/16 in the url it just crashes.
What am I doing wrong?
In reply to Chris's reply below. All of these are optional.
Structure of url is like so:
myasite.com
myasite.com/uk (if set, This will always be text and always 2 chars long)
myasite.com/uk/london (if set, This will always be text, this will be any char length )
myasite.com/uk/london/16 (if set, This will always be integer and any char length)

Answer 1

Your regex is incorrect. Your ^/?$/?$ says the request can have 2 /s only, each is optional. You also aren't using any capture groups so $1 and $2 have no context. Here's a regex that would work for your provided example:
^/(uk)/(\d+)$
If uk can be any 2 lowercase letters you could use:
^/([a-z]{2})/(\d+)$
You can use regex101 to see how your regexs will function.
https://regex101.com/r/VyJE9d/1 (your rule)
https://regex101.com/r/VyJE9d/2
The right side of the page gives explanations.
As a rewrite rule:
RewriteEngine On
RewriteRule ^/([a-z]{2})/(\d+)$ index.php?reg=$1&id=$2 [L,QSA]

Answer 2

All you need to use is this in your .htaccess file:
RewriteEngine On
RewriteRule ^([^/]*)/([^/]*)/([^/]*)$ /index.php?reg=$1&area=$2&id=$3 [L]
This will leave you with your desired URL of: http://myasite.com/uk/london/16. Just make sure you clear your cache before testing this.

Answer 3

RewriteEngine On
RewriteRule ^([a-zA-Z0-9_-]+)/([0-9]+)/?$ index.php?reg=$1&id=$2 [L,QSA]
We are rewriting $1/$2/ and $1/$2 to index.php?reg=$1&id=$2