Mod_rewrite cannot use a variable in a regex pattern. The .htaccess directives are not a scripting language...
RewriteCond $1<>$3 ^<>-([0-9]+)-([0-9]+)-some-think$ [OR]
RewriteCond $1<>$3 ^some-one-<>/pag-([0-9]+)$ [OR]
RewriteCond $1<>$3 ^a-z+-<>$
RewriteRule ^([^\-/]+[\-/])*(skoda|bmw|mercedes)([\-/].+)?$ index.php?a=$2 [QSA,L]
Here, the RewriteRule pattern is evaluated first (See Apache mod_rewrite documentation "Rule Processing").
If the pattern matches, then whatever comes before "(skoda|bmw|mercedes)" in the requested URL-path is placed into local variable "$1".
Whatever follows "(skoda|bmw|mercedes)" is placed into local variable $3.
The value of the requested URL-path matching "(skoda|bmw|mercedes)" is placed into $2.
Then each of the RewriteConds is processed to check that the format of the requested URL without the "(skoda|bmw|mercedes)" part
is one of the formats to be accepted.
Note that the "<>" characters are used only as a separator to assist correct and unambiguous parsing, and have no special meaning as used here. They simply "take the place of" the variable-string that you do not want to include in each line. You can use any character or characters that you are sure will never appear in one of your URLs without first being URL-encoded. I prefer to use any of > or < or ~ myself.
Note also that the RewriteRule assumes that the "(skoda|bmw|mercedes)" substring will always be delimited by either a hyphen or a slash if any other substring precedes or follows it. I am referring to the two RewriteRule sub-patterns containing "[^\-/]" ("NOT a hyphen or a slash") and "[\-/]" ("Match a hyphen or a slash"). This greatly improves efficiency of regular-expressions pattern matching, so use this method if possible instead of using an ambiguous and inefficient sub-pattern like ".*" (Match anything, everything, or nothing").