I'm trying to understand Alchemy better. When I go to an Alchemy Table I have a list of potions down the left side. Some are greyed-out and some are highlited white. If I click on a white one I start selecting ingredients from the ones it provides me. If there are 3 or more listed I can click on 2 of them, but it states that there is room for an optional 3rd ingredient. Most of the time when I click on a 3rd ingredient nothing at all changes about the potency. Why would I add the 3rd ingredient? Also, can I only mix from that list of white potions or are there other possibilities?

Thanks

Three ingredients makes it easier to have multiple effects in one potion.

The "potions down the left side" you mention are actually ingredients sorted into the effects that you know they have. Ones that are greyed out indicate that you don't know of any ingredients with that effect.

If you select the "ingredients" option, you'll see ALL the ingredients you are packing around and can freely mix up to three them to produce whatever you want- often no result, potentially a potion or poison with up to 6 different effects, although there's currently no set of ingredients that allows more than 5 effects. Mixing up potions this way is a great way to learn new effects (and make tons of high value potions, and earn xp for alchemy), but it helps a lot to use a tool like the one in my sig so that you don't waste lots of ingredients figuring out what mixes with what.

You usually need only 2 ingredients. But sometimes by adding another one gives you a different potion.

What you must do is:

Click on INGREDIENTS (top of the left list). If you have an ingredient with only 2 effects revealed (like Beehive Husk - resist poison / fort. light armor) click on the ingredient then choose one in the left list (in that case try Fortify Sneak) and you'll get some ingredients like Frost Mirriam, Hawk Feathers, Purple Mountain Flower. Click on one of these ingredients. Like Frost Mirriam. You'll have "Potion of unknown effect". So make this potion and it says: Discovered effects: Fortify Sneak (Beehive Husk). Now your Beehive Husk has 3 revealed effects and you have increased your alchemy. And so on... You can also improve your skill by creating expensive potions. All kind of poisons like damage health, stamina etc. are more expensive and increase your alchemy.

Now, by adding Hawk Feathers to Beehive Husk and Frost Mirriam (Potion of Fortify Sneak) you get another effect like Increase Light Armor. So now you have a potion with 2 effects. If you add instead Purple Mountain Flower you get a second effect with Resist Frost Damage. These potions will be more expensive and you'll increase your skill. Try.... Try....