Baldur's Gate 3

So you have Shadowheart, Wyll, and Astarion, at least. You missed Lae'zel? Did you get Gale?

There's a Barbarian character that you can pick up a bit later than the other companions. Make sure to get her before leaving Act 1, she makes a great front-liner.

There will be more companions to get in Acts 2 and even 3.

If you have Withers at your camp, you can also respec companions or get a hireling.

You can respec any character at whithers in your camp. Also as other have said circle of the moon druid can bear form to get an extra 60 hp (half from each cast) circle of the spore can spawn minions that can tank the hits that would other go towards your team mates.

It sounds like your problem isn't just that you don't have a tank but that you're solely relying on Shadowheart for healing. In my run I'm doing an ancient paladin so I've got lay on hands plus the heal aura to subside her healing.

When you level up you can also multiclass to get some desirable traits from another class to help your comp.

You can change classes once you meet an NPC early on. Explore around the starting area, and you'll meet someone that'll show up at your camp and let you respec anyone.

Also, druids can get tanky if they go the wildshape route

Warlock, Druid, and Cleric can be built into tanks pretty easily via Feats and Subclasses. Heavy Armor proficiency solves a ton.

That said you did miss Lae'zel, the Str Fighter; and Karlach the Barbarian.

There's some additional late game companions in act 2 and 3 but not too much. A lot of casters (including druids) will also gain elemental summons as spells in later levels. The summon itself can be a tank for you. You can also respec characters at Withers in your camp. You want to be careful changing origin characters classes, but subclasses are fine. Clerics can be great tanks, especially in the life domain. For example my current character is a cleric/monk with an armor class of 24, almost ever attack misses them.

EDIT: removed my first sentence, it mentioned the class of a later game companion which may be considered a spoiler to some.

If you are playing a shape shifting druid your character can easily tank. Especially if you pick up a few levels in Barbarian. Rage stays active after shape shifting and a raging bear is one of the tankiest builds available.

Without knowing how far you are it's difficulty to say if any companion are still available. Except for Shadowheart and Asterion I found all other companions fairly easy to miss in a blind playthrough. So you might want to read up on who they are and where to find them.

To actually answer your question, since no one else did ... yes, there is one additional melee character you can get in Act 3, but he's not the easiest character to find. It'll be a slog to get that far without a tank.

Since you have a druid, you could try using a combo of animal form and summons to tank for you. I've found elemental summons are pretty durable, but won't do as much damage as a real tank.

Otherwise yes, you could respect a party member to tank.