add page name before ^
RewriteRule ^category/([a-zA-Z0-9-]+)$ category.php?category_slug=$1 [NC,L]
RewriteRule ^subcategory/([a-zA-Z0-9-]+)$ subcategory.php?subcategory_slug=$1 [NC,L]
RewriteRule ^products/([a-zA-Z0-9-]+)$ products.php?product_slug=$1 [L]
and write your url for category page as example category/mobiles/
UPDATE
The original URL:
mysite.com/category.php?category_slug=mobiles
The rewritten URL:
mysite.com/mobiles
RewriteRule ^([^/]*)$ /category.php?category_slug=$1 [L]
The original URL:
mysite.com/subcategory.php?category_slug=mobiles&subcategory_slug=samsung
The rewritten URL:
mysite.com/mobiles/samsung
RewriteRule ^([^/]*)/([^/]*)$ /subcategory.php?category_slug=$1&subcategory_slug=$2 [L]
The original URL:
mysite.com/products.php?category_slug=mobiles&subcategory_slug=samsung&product_slug=galaxy-note
The rewritten URL:
mysite.com/mobiles/samsung/galaxy-note
RewriteRule ^([^/]*)/([^/]*)/([^/]*)$ /products.php?category_slug=$1&subcategory_slug=$2&product_slug=$3 [L]
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