What makes monks competitive with Paladins, if they are?

[Eldariel]

Question: if a boss is fighting 3 PCs, is it better to have:

-3 PCs who can equally be targeted by the boss

or

-2 PCs who can equally be targeted by the boss and one who can stay out of harm's way while the boss attack while still dealing damage to them when it's time to act

Depends on the PCs and the boss. If the boss is capable of multitargeting at long range, of course as few as possible. Most bosses though are very limited in that regard though: generally they will have some very avoidable AOE and mostly high powered single target effects.

In this more typical case, if the 2 PCs are better at defeating the boss than the 3rd one, definitely 3 who can equally be targeted. This at least gives the boss the chance to mistakenly attack the lesser threat. OTOH if the lesser threat automatically removes themselves from the threat pool, the PCs are essentially automatically optimising the boss's actions for the boss to defeat the greater threats first and then the lesser threat.