Firstly, the term "short stack" only really relates to a pre-flop description of relative stack sizes of all players. It doesn't have a reasonable use after the action has started. That said, a player may be "short stacked" post-flop but that's entirely down to what action has occurred.
Specific to the example, player A can only call the All-In for $7, although his total amount staked is $10, equaling that of player B. Opposing players still to act can only call the difference between their current staked amount and what opponents have bet, assuming they are closing the action. Otherwise, they can bet/raise up to their total stack assuming another player still to act can match it. You mention "before betting" and "after betting" and i'm not certain of what you mean, but it seems irrelevant as the game is sequential. Further action only exists after any action state. The term short stack is only a description and has no real use in game mechanics.
As for the side pot issue, player A is involved in ALL pots as his bet covers everyone. That said, in your example:
- players A, B, & C contest the main pot. ($27)
- players A & B contest the only side pot. ($2)
In a little more detail, player C can only win a multiple of his total staked amount ($9), thus the $27 main pot, or 3 players in that pot for $9 each. Players A & B can win the main pot and the side pot, where the side pot consists of the extra money staked above what the lowest amount staked was (i.e. above $9). Player A only has to match this extra amount of $1 (i.e. difference between what player B & C staked), thus 2 players in the side pot totaling $2.
After all bets are matched, player A still has $1 behind which is not in play (i.e cannot be lost or won with), and players B & C are All-In.