Starting a new Waterdeep Dragonheist campaign. We have three players, all of whom are fairly new, but very motivated to dive into the intrigue and faction play of the adventure. Two of them have even chose criminal/political backgrounds, and are planning to use those to help the party infiltrate all aspects of society. Very encouraging! 

However, they also consist of two rangers and a rogue, all of whom picked ranged weapons and spells. As I try to refrain from ever limiting my player's choices, I consider this a perfect challenge as a DM. My question is how do I plan encounters and balance combat with the lack of higher health or melee damage characters? I'm trying to brainstorm ways to vary the rooms of the dungeons to favor more types of play, but these are newer players who don't always think tactically. In a previous game we played, combat generally devolved into standing around the enemies and hitting them over and over. Also at that point, they hadn't really attempted to optimize or upgrade any of their equipment, so they would constantly fail their attempts, or do very low damage. So as a followup question, how do I encourage a bit more strategy? I would prefer no TPKs this run

The proper strategy for two rangers and a rogue is sneaking around ambushing their foes, so try to set things up so that style of play is actually possible. I don't have great advice for how to get them to think about tactics; you can give them suggestions but it's up to them whether they pay attention.

Accept that the players need to be smart.  If they don't play smart, well, some parties just don't have the right composition to survive. Give them the option to succeed, but don't make reality bend to give them victories even if they play like idiots.

If they aren't 2nd level yet, then after a few fights one of the rangers might decide to choose a melee fighting style to keep the enemy at bay.

Or they could hire a warrior follower to act as their meat shield :)

While they may be ranged combattants at base, they can still be effectve in melee with finesse weapons when needed being DEX based classes.

You'll likely need to give them information early - don't wait until they are in a place t otell them about it if they can already see it. Being ranged means they don't want to walk into a bandit camp before noticing it's there.

If players choose an unusual or skewed composition then just run with it. Don't modify anything, don't give them special treatment - if they figure out that it's really beneficial that they ambush enemies, then let that be their decision.

The more you don't change anything for them, the more special it will feel when they work out strategies, come up with plans etc. Yes, there are times when they may wish they had a high STR or CHA character around, but that's not the hand fate dealt them. They can figure things out as they go.

Don’t do anything at first and see how they deal with it. Maybe one ranger will go beastmaster and the other will go drake warden, problem solved. 

Take notes on what they do and when they do it. Allow them to make mistakes that are not lethal. Let them all be unconscious and their stuff taken. Then talk with them.

If it ends poorly talk to them about it. If it ends well talk to them about it. Ask them after the session what they think they did well. Allow them time to talk about what went on with the tactics they used. Listen to what they say. Ask somewhat leading questions on how it could have been done. Let them think about it.

Then try a similar situation the next session and see if they change tactics. If they do then there is hope. If not then adapt the best you can. 

You can train them by the discussion out of game OR

Have them meet up with an in game NPC veteran and have him tell stories of how he did it when he was young.

Maybe the thieves guild will mandate some tactics training if they want to work in the city

Maybe they meet a ranger in the wilds and have him explain how he used strategically placed traps to funnel the goblins into a shooting gallery for his lone bow and animal companion.

Have them happen to be on a hill overlooking a skirmish that has mostly ranged combatants and describe the things that happen on the battlefield.

Use the game world to help reinforce good tactics and discourage the bad. If it is bad tactics that keep working then they are good tactics by definition.

All great advice, but IMO the biggest thing is you shouldn't be custom building every encounter to play into their hands.  Same would apply to a melee party with no ranged, there WILL at some point, be someone at range picking at them and they need to find a way to deal with that.  Rangers are better melee than many folks give credit for.  In Studded Leather, a high Dex Ranger can surpass or at least meet the AC of a plate wearing fighter.  They are not useless or handicapped so far as melee options go.

I would personally ease them into it, by having some early scenarios enable them to ambush foes and work from range, but also having some enemies rush them (not all enemies are stupid lol) so kind of force some direct melee combat.  If they complain, simply explain that the Orc with a club wasn't impressed with the skinny elf shooting him so closed the gap.  Also explain that this WILL happen, likely many times.

No real issue with character that prefer ranged, but the classes you have are certainly capable of doing to melee fighting with no major drawbacks.  So far as gear goes, I have always made a point to have gear drop for my parties as required.  They do look for magic shops and armorers, but we are pretty seasoned D&D-ers now, so that's to be expected.

Some sound advice here. I would echo not bending the world to suit the PCs. The world should react to the PC input, not dictate a particular style of input. 

Let the players learn how to operate inside the game with their PCs. Solutions to obstacles and challenges is a player function, not a DM function.

I disagree that you shouldn't adapt the campaign to the PCs. You don't want the PCs to always win, but if the PCs want to play a stealth game, there's nothing wrong with letting them do it. Dragon Heist isn't by default structured as a stealth game, but you can certainly transform it into a heist caper.

One thing about a stealth-focused game is that under default rules, a single failed stealth roll has a much worse consequence for the party than a single failed attack roll has for a normal party.  You might want to require two or more failed stealth checks to get noticed in order to give the party better odds at successful sneaking.  Especially against critical encounters.

Ranged characters are often easier to keep alive because they just need to keep distance to avoid melee damage. 

That's not as easy as it sounds when you don't have anyone who wants to rush into melee and distract things from charging you.