will insignia nullify ascension?

All of my armor is infused and has a -3 physical damage stat. If I add an insignia to it will it get rid of that stat?

It will remove the -3 physical damage but will not remove the infused bonus.

If your referring to a Knights insignia for the -3 yes adding another insignia to the piece with the Knights on it will replace it. If you place it on a different piece of armor that doesn't have an insignia it won't replace it.

If your referring to a sup absorption rune an Insignia won't replace that.

Hm, so what's more beneficial--- negative 3 physical damage or +15 health? I found a survivor insignia

It shouldn't do, unless there is some 'feature' I've not come across. If the -3 damage is inherent in the armour, the insignia will not affect it. If there is already an insignia (which provides the -3 damage) then adding another insignia to that piece of armour will remove it.

Insignia's don't change infusion.

The 15 health is more beneficial because the -3 physical only activates if the attacks hit your chest. The 15 health can help you stay alive longer in case of degen and makes you more resilient to armor ignoring spells (albeit not much, but every little bit helps).