I've got a group of 6 players running in a campaign.  I've been using "deadly" sized encounters based on CR numbers.

What I've been finding is that one or two will go down to zero only to be brought right back up and almost full health in a round or two by their buddies.  Unconsciousness seems to have little to no consequences due to this.

However, I am concerned about adding additional CR to the situation and risking a TPK, not my goal.

My goal is to have a tough fight, where players feel like they are actually risking something.  One where a death saving throw or two have to be thrown maybe, or there is some other sort of serious concern.

Any ideas on this? (they are currently level 3, so maybe it gets better with more HP, though small little heals the player pops back up, only to be smashed back down like a whack-a-mole.

In any case I could use some advice.

2 thoughts.

1 - deadliness increases if the encounter is one of a sequence, particularly if the characters have blown spell slots and abilities without the ability to rest. Use attrition to your advantage!

2 - tactics. Target healers, and then see how the party comes back up mid-fight.

Think about environmental hazards as well. Can't reach your fallen comrade as easy when a fissure opens in the floor to separate you, or the spider webs covering the floor slow you down. Certain enemies may swarm the fallen, wanting to feed while the body is still warm. Others may actively hamper efforts to reach the fallen, and focus on intercepting help before it gets too close. Looking at combat through these lenses may help mitigate such problems.

I'm currently using a variation of the "gritty realism" rules, and I've eliminated healing potions, in favor of a custom healing spell which has a ritual aspect & ( expensive ) material components.

This means that:

       - Spell slots regenerate much more slowly ( instead of the Long Rest being a full 7 days, I'm awarding 1/7th of the effects of a Long Rest per day: 1/7th of hit points, 1/7th of hit dice, and 1/7th of all spell levels - sort of teetering on the edge of the "spell points" variant rules there).

       - Potion based healing ( or its equivelent ) is only something that can happen between combats.

      - I try to play monsters and opponents as intelligently as possible and is realistic; they can't have perfect knowledge of the party, but they can act on what they can see and deduce. Squishy spell-casters that unload offensive spells are often targeted.

It makes the consequences of reckless combat a little more immediate for my party.

Frankly one of my problems is that I've got two druids and a priest, each able to fling a heal over distance.  Someone mentioned healing potions being a problem, but I'd rather have healing potions than as many little ranged baby heals.  With a healing potion they at least have to get next to their dead buddy, which presumably puts them near the enemy as well. And as the GM I can control how available the potions are.

I actually think @JCAUDM's suggestions are good ones... soften them up prior to the big fight...make them burn through spells so they have to conserve them.  If I do it often enough they'll learn and stop flinging heals immediately.

Another suggestion I've read is to throw a ton of trash their way (Kobolds) so that their actions are burnt through without being able to hit everything...of course, same balance applies, too many and it becomes ungainly and can be just as deadly.  The advantage to this approach is with a thousand paper cuts it takes a while to kill someone, so they might get the picture and run for their lives... >:)

Many opponents are much more deadly than a few big ones, for the same CR; it's the action economy.

How about a mirror match?

Every party member gets cloned, and the player party has to fight the clone party. Both sides have the same abilities, same stats, same items, and same of everything. You don't get a more equal matchup than that.

Or you could hit them with a swarm. Once someone goes down, put enemies between the downed character and the rest of the party. Make it a challenge for them to heal their companion back up. If someone tries to run past every enemy to their companion, they'll take a lot of damage on the way.

If not, you could hold an encounter in an area where you halve all forms for healing. Call it a "zone of undeath" that radiates negative energy and interferes with divine spells.

Alternatively you can have an encounter that the players are supposed to run away from. Let them find a lone hill giant walking around, and if they engage it in combat, it will signal its friends who are nearby. The party could probably handle a single hill giant for a few rounds, but how about 4 more? Seeing one of their party members take 15+ damage on a single hit and knowing that 4 other similiar enemies will arrive in a few rounds will make your players reconsider their chances of winning. Do make sure that your players can outrun the giants somehow though, as the giants have 40 ft. speed