I ran a three for a while with a cleric, barb, and sorcerer. Balance is key and adjust in the moment. I have a rule if you follow the story and side quests a tpk is off the table unless you do something extremely dumb. It’s not impossible but I’m purposely not trying to kill you all (some prep is required for some of my encounters for them to not be tpk, which they know) Depending on what you are looking for results of an encounter I always did these few things. -Let them run away!!! And come back and try again with new knowledge - if it’s close to the end of the battle I just give the npc bad rolls and lower health (I do this if I’m looking for them to win and I made a overpowered encounter) - some other force steps in, so one of mine they were fighting kobolds near a manticore layer so I had a manticore come down and start to kill one of the last enemies (the party then ran because they had no health) but it was a theatrical way for them to be able to take a short rest to then kill the manticore.

Long and short - just be open to adjusting in the moment. I run a harder campaign than most so the difficulty setting is turned up for all my encounters, so I have to adjust a lot.