You are only capturing one variable in your rewrite rule.
You need something like:
RewriteRule ^([A-Za-z0-9_-]+)/([A-Za-z0-9_-]+)/([A-Za-z0-9_-]+)/?$ process.php?width=$1&height=$2&third=$3 [QSA,L]
or, a bit shorter:
RewriteRule ^([w-]+)/([w-]+)/([w-]+)/?$ process.php?width=$1&height=$2&third=$3 [QSA,L]
(the w
word character includes letters, digits and underscores)
I have made only the ending slash optional so this rewrite rule would only do something if there are exactly 3 variables.
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