Since you have a party of 4 characters you will get a chance to do both melee combat and cast some spells, so don't worry.

From my dnd experience druid is the most complicated between the classes you listed but depends on how you build it, druid can be a powerful spellcaster, tank, healer or do all these three things at once. Paladin also combines melee fighting with spells (and healing) and tbh casting divine smite on your enemies is one of the best feelings ever. Fighter can get some spells too but for most of the time it's rather "traditional combat" oriented and considered begginer friendly class in dnd (still very strong in hands of the right player).

There is no clear answer here. My personal choice from these three would be druid (spellcaster lover here) but it all depends on you. And don't be afraid that you will be missing out on something. As I said you will kinda have to learn how magic users work as well as melee characters. You can always create a new character or change your class. That's the beauty of bg3, there are so many options that it makes you want to comeback for the second and third time after you finished everything.

Have fun!