Going by the roles listed in this answer I would say that you have a fairly balanced party that covers most roles adequately.
A person who can heal
This is one area where you might be lacking a little. The Paladin gets Lay on Hands starting at level 1, and they get spells that include Cure Wounds at level 2. The Ranger also gets spells like Cure Wounds at level 2, however they have a limited number of spells known so they probably can't devote too many to healing ones. I would recommend having someone with the Healer feat if at all possible, or proficiency with a Herbalism Kit to make your own potions.
A person who can do area of effect damage
This is covered by the sorcerer. They can start at level 1 with Burning Hand or Thunderwave, and have more options as they level up.
A person who can tank up damage well
The paladin has this covered, with d10 hit dice, heavy armor and a shield they should be difficult to hit and if they are hit they take a lot of damage before they are removed from the fight. The ranger is also decent at this, also having d10 hit dice and a decent AC.
A person who can deal lots of damage
This can be done by everyone fairly well. The paladin has Holy Smite to boost his damage, sorcerer has spells that do a good amount of damage and metamagic to improve them as well, and the ranger gets the spell Hunter's Mark.
A person who can bypass obstacles easily
This one is a little vague since an 'obstacle' can be just about anything. However you have most of the most common ones. For a party face the sorcerer should be able to do well (and possibly the paladin depending on where they put their stats and proficiencies, though being a kenku their ability to help here may be limited). The ranger should be able to be a good scout with Stealth and Perception.
There are a few places they might struggle however. One being traps and locks since nobody has thieves' tools proficiency their main answer is going to be to brute force it. And the other is knowledge based challenges since they might not be proficient in many of the requisite skills (acolyte has Religion, however the rest might not be covered), and unlike things like being a party face or scout the party probably won't have the intelligence to do well with these checks if they don't have proficiency in them.
A 4th member
My recommendation would be either Bard or Wizard, with a background that gets proficiency with thieves' tools (Criminal or Urchin from the PHB or Urban Bounty Hunter from SCAG), or a herbalism kit (Hermit from PHB). Both of these classes help with some of the skills you are missing and also add some battlefield control that while it isn't a necessary role is helpful. Half-elf or Human would help with getting skills, however this isn't super relevant.
For the Bard they should pick one of the ways to get thieves' tools, since they get spells can heal so they don't have to worry about that. I would say something like Sage to help pick up the knowledge skills. And with Jack of All trades at level 2 they will be somewhat competent at most things.
For the Wizard I would probably choose the Hermit background, because they have spells that can fake the usage of thieves' tools (things like Knock aren't perfect, but I think the party could use more healing). For them while picking up proficiency in some knowledge skills is useful it isn't as necessary since they will have the intelligence to pull them off a lot of the time any way.