Advice needed: encounter balancing for a party with 3 paladins

Hi there, fellow DMs!

I'm a fairly new DM (as in: I have around 20 sessions behind my back), and while my players seem to be enjoying the campaign, I've run into a bit of a problem.

Namely, that the three godsdamned paladins are trivializing most combat encounters.

They just leveled up to level 8, but even at level 7:

• Attack rolls against them? LOL, CR 7-9 enemies usually have +6-+8 to hit at most; they will miss the paladins (and the cleric) in plate armor + shield 60-75% of the time.
• Saving throw abilities and spells? Fuck me, aura of protection, everybody gets +2 or more to all their saves.
• Even if a spell slips through? Ancients paladin. Whoever came up with the Aura of Warding at WotC deserves a kick in the head. Everybody near the paladin takes half damage from every spell (quarter if they make the save) because balancing encounters is soooooooooooooo easy!

And that is just their passive abilities. There's of course the usual issue of smites (the three of them can easily deal 24d8 damage in one turn, that's 108 on average - and that's without accounting for crits or them stacking a smite spell on it too). Ranged enemies? LOL, orbital laser goes BZOT! (Moonbeam) Or they'll just leave them to the ranger, cleric, and the warlock. And if they still get banged up, they have 105 HP of dedicated healing between them (plus the cleric and the ranger).

Is there any way to make combat encounters challenging for this party besides trying to overwhelm them through action economy (it's a party of 6, so that would take a shitton of monsters and turn the combat into a slog), finding a way to force them into 6-8 encounters between long rests (wouldn't do anything about the passive abilities but it would at least curtail the smite-nukes), or turning the game into Dark Souls with every monster being a horrible damage sponge that can one-shot any player on a hit?

Because at this point I'm afraid that anything shy of a tarrasque would be a minor inconvenience at best instead of a challenge or a boss.