The backing group size on Kusama (and Polkadot) has been set to 5 (configuration->activeConfig->maxValidatorsPerCore). This proposal is aiming at reducing the group size to 3.
Currently, when a parachain produces a block, it requires 2 out of 5 backing validators to verify and sign it in order to back it and move forward. With 2-of-3, we'd guarantee that backing quorum is a group supermajority, so there're not two competing 2/5 backed candidates. And one extra validator is allowed to be slow. Smaller backing groups should also contribute to distributing the load of approval-checking more evenly.
There's a small risk, however, that with slow/unresponsive validators, parachain slots might be skipped more often with 2/3 instead of 2/5. This should not be an issue in practice since validator groups rotate every minute.
Kusama and Polkadot have a group of validators that back up blocks produced by parachains.
Currently, the group size is 5, but there is a proposal to reduce it to 3.
With a group size of 2 out of 5, two validators are needed to verify and sign a block.
With a group size of 2 out of 3, a supermajority is needed to back a block, and it can handle slow validators better.
Smaller backing groups can distribute the load of approval-checking more evenly, but there is a small risk of skipped parachain slots with 2/3.