Still kinda new to alchemy, but what point does the optional reagent have? Tried doing healing potions but same effects with two reagents as it is with adding another heal ingredient. Or does it only matter if you try and add another effect, like add a stamina restore reagent to a healpot?

Third ingredient is really just for experimenting, or when you are trying to make a potion with multiple effects.

I can't remember exact effects of reagents, so here's a few entirely made up examples.

Reagent 1: Fortify Health, Restore Health, Regenerate Health, Damage Stamina
Reagent 2: Restore Health, Resist Cold, Invisibility, Resist Disease
Reagent 3: Resist Cold, Resist Fire, Resist Shock, Resist Poison

Example 1:

Mixing Reagent 1 and 2 will give you a potion of Restore Health
Mixing Reagent 2 and 3 will give you a potion of Resist Cold
Mixing Reagent 1 and 3 will fail
Mixing Reagent 1, 2 and 3 will give a potion of Restore Health AND Resist Cold

Example 2:

You don't know the effects of the above ingredients.
Mixing Reagent 1 and 2 will reveal that both have Restore Health
Mixing Reagent 2 and 3 will reveal that both have Resist Cold
Mixing Reagent 1 and 3 will fail
Mixing Reagent 1, 2 and 3 will reveal that 1 and 2 have Restore Health, and 2 and 3 have Resist Cold.

Pro of making a potion with multiple effects:
-Uses 3 ingredients to get 2 effects instead of 4 ingredients.

Con of making a potion with multiple effects:
-Have to get both effects at once, so if you don't want resist cold, and only want to restore health tough cause they're combined.

The third reagent allows for potions and poisons with up to 4 effects.

Thanks for very nice information. Much appreciated!

Basically... it is for the true 'unknown' potions.... meaning you may be mixing with 1 or 2 ingredients, having no idea if they will create something or not... and you throw in the 3rd as a just in case.
Sometimes the Optional item saves the day, so to speak, and is one of the real necessary ingredients. For 'experimentation' ... or Plan B