Encounter balancing.. help?

The party is level 3, consisting of 4 players, and 2 npc (levels 1 and 2)
Barbarian - 20ac, +10 to hit
Kineticist - 20ac, +9 to hit
Druid - 16ac, Class dc 19
Cleric - 17 ac, Class dc 19

NPC are fighter level 2, and Swashbuckler level 1
The encounter was 3 Husk Zombies.
17ac, 55hp, +11 to hit. Permanent Slow so they only have 2 actions. +7 Fort and Will save, +9 Ref.

The zombies are about to lead to a tpk and I feel like I am doing something wrong with balancing encounters. They have felt like the players just walk through them, or are going to die. There's not been a healthy middle and I am not sure how to achieve this. A moderate encounter is 80xp, these 3 zombies are 90xp (30 each) so it falls in with it. Party level -1 are standard monsters according to https://2e.aonprd.com/Rules.aspx?ID=2716&Redirected=1

Anyone willing to help me out?