Can Sergei Kovalev defeat both Stevenson and Hopkins?

I say yes, and easily.
Sergei Kovalev should be considered the best Light Heavyweight in the division and his competition proves that. Far better record than either Adonis and current BHop.

Darnell Boone has lost to Ward, Vera, Pascal, Lara, Edwin Rodriguez yet none were able to knock him out, 2 months after he fought Kovalev to a SD loss he was KO'd in the first round by Periban, could it be the lingering damage from Kovalev weakened his chin for Periban, possibly.
In the rematch he was destroyed by Sergei in 2 rounds, meanwhile after 2 KO's and 2 matches with Sergei, it still took Adonis 6 rounds to beat his former conqueror. Boone says Kovalev is the best he has fought better than Adonis and Ward.

Gabriel Campillo is a very underrated fighter who rightfully should have victories over Cloud, Shumenov twice and Murat. He got his belt by beating Hugo Garay who had just beaten Juergen Braehmer. He was krushed in 3 by Kovalev.

Cleverly was coming off legit UD's over Bellew and Mohammedi, and a KO over Murat, top prospect destroyed by Kovalev just over 3.

Sillakh was on a roll until he was TKO'd by Grachev who at the time was on a roll as well, Kovalev brutally KHTFO in 2.

Lionell Thompson undefeated KO'd in 3.
Cedric Agnew undefeated KO'd in 7.
Roman Simakov had 1 SD loss unfortunately killed by Kovalev in 7.


On the other hand Adonis Stevenson got his belt by beating Chad Dawson who was stopped by Andre Ward, who stops virtually no one.
He stopped Tavoris Cloud but didn't KO him.
He KO'd Bellew who was beaten by Cleverly and barely beat Chilemba, Bellew didn't got the credit for his KD of Stevenson in their fight.
It took him 12 rounds to stop Don George who took a lot of punishment but took his power well.
Fonfara went the distance and dropped him, exposing his chin again.
Darnell knocked Adonis out cold in 2.

Fonfara stopped Campillo in 9 after the Kovalev match.
Fonfara dropped Adonis, Kovalev hits way harder than Fonfara, and Kovalev took Boone's shots far better than Adonis. Better power and chin.

Hopkins is amazing for his age, but he struggled at the beginning of the Cloud and Shumenov matches. Both of whom Campillo should have been given credit for beating. He struggled with Murat for a few rounds, Murat had just been stopped by Cleverly, he didn't stop him.
But before that he looked bad against Dawson and lost, before that got dropped by Pascal twice in the first fight, did better in the second, all close fights.
Hopkins as an ATG but he has struggled recently against top opposition, who don't have close to the punching power of Kovalev.
If anyone can give Kovalev trouble its Hopkins, but Kovalev has scary power far greater than Hopkins recent opposition, and I'm not sure how well Hopkins 49 year old body would hold up under Kovalev's body shots.
The reality is, the amount of time it takes Hopkins to figure his opponents out is the same amount of time it takes Kovalev to stop them.

How long would Cloud, Bellew, Fonfara, Murat, Shumenov, Dawson, George last against Kovalev?

Stevenson's awkward southpaw stance may trouble Kovalev at first, but Sergei's chin would hold up at least as good as Fonfara or George's, Kovalev has subtle head movement, takes slight steps back to take the sting off alot of shots, and he would get to him eventually.
Hopkins is great at countering and smothering predictable fighters, unorthodox fighters like RJJ and Calzaghe not so much, Kovalev hits you any and everywhere he can land, Hopkins body would eventually give out, he's getting too old to take a sustained beating from someone with Kovalev's power.

 

He's faster than Murat, Shumenov Dawson and Cloud maybe even Pascal, he's faster than your giving him credit for.

 

About the same speed as Kovalev, and do you really think with that study Dawson didn't put everything he had into that shot. Dawson is fast but he wasn't that fast with Hopkins though.