What is a good amount of damage for this spell?

I've made a post about this before, but that threads as dead as a lich so I'll post about it again.

I have a homebrew spell called Palus's Endless Bog, a Level 3 spell that currently creates an area of marsh that is a 20ft emanation (range 150ft) of difficult terrain that makes any creature that starts their turn there make a CON Save, dealing 3d6 Poison DMG + giving the Poisoned condition, or only half damage on a success.

Part of me is thinking that 3d6 is too low, so I want to hear thoughts. Btw my players will be able to use this spell once the creator of it, Palus (a friendly arch-hag) teaches them it at level 5. Should I keep the Poison Damage at 3d6 or make this attack more threatening?

3d6 seems good. Spirit guardians is a good comparison, and that does 3d8.