Just a party of 3? Definitely a caster. My advise would be either Bard, Druid or Sorcerer. If you want to double down on healing, Cleric would be decent as well.
Bard is the best buffer and debuffer in the system. Will make both the Swashbuckler and Rogue better at what they do and cover decently for the wholes your party currently has, mainly AoE and spell utility like flight or dealing with invisible enemies.
Druid is good for a more offensive approach. It is also more independent than Bard. Mostly damage and healing, but it also comes with some buffs, debuffs and utility, just worse than bard in these last 3.
Sorcerer is easier to play than the other 2 and it can kind of cover the roles of both depending on your bloodline (or just go for Arcane and be a better Wizard). Its main appeal are its 4 slots per level, so you get more fuel in the tank.
One of the 2 games I'm currently playing has both a Battledancer and a Thief as well. We have a Precision Ranger (me), a Tyrant Champion and a Demonic Sorcerer on top of that. Party works really well, but we definitely need the caster to fill the holes, so I figure it will be the same for you.
Depends on how long the game is going to take. If you are going to stop at level 10 or so, Warpriest is quite good. If you are going all the way to 20, I would advise to go for a Cloistered Cleric instead.
If AoE damage is a concern of yours, cloistered will be a bit better, but almost any other caster would be better at solving that than a Cleric.
I think Bard would make a great 3rd man to your party OP.
The buffing, healing, and debuffing you can do will definitely help.
I assume when you say the rogue is geared for healing, you mean they have the medicine skill and plan on getting the appropriate skill feats.
I would suggest that you don't plan on going into healing much, but knowing spells like Soothe for healing mid combat would be good.
Otherwise dropping inspire courage and the occasional debuff spell can go a long way I think.
Definitely go with Bard. Best spellcaster in the game.