Dragons are cool, but grossly underpowered in 5e. They deal little damage outside of their breath, which frankly doesn't recharge that often. They don't have very many hit-points and they're not that difficult to hit. (assuming +4 stat mod, +2 proficiency, and at least a +1 from some other source, you're hitting a 17 a minimum of 50% of the time). Worse are dragons in caves. Dragons that can't fly away when they're cornered are dead dragons. In one-on-one fights they are also majorly lacking in actions. The party gets anywhere from 5-8 actions, while the dragon gets one.

So yes, add some lair actions and if not lair actions, some kind of hazardous terrain or other dangers the party has to face. Also: add some minions. These dragons are vying for this dungeon, surely they're not doing it by themselves.

Beyond that, it depends on how skilled of tacticians your party is, spreading out, buffing before the fight, controlling the battlefield will always up a party's CR, and of course how well built your party is. In 5E even a moderately optimized group can "punch up" easily. And by "optimized" in 5E I mean nothing more than +4's or +5s in their main stats, and having a diverse party. (Fighter, Rogue, Cleric, Wizard, *other*).

In my experience, CR is linear and players are exponential. I would generally say subtract 2 from the CR for each tier above the first to get a true estimate of what level players can take on your challenge.

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