When I select this feat wile making a PC it gives me 3 spell options instead of 2 

You can keep adding more spells if you come across them in-game, so that third dropdown is for that. If you were to pick something, a fourth dropdown would appear, etc etc.

As mentioned, the first 2 are what you start with. The third is for what you pick up later, and adding it keeps making a new slot.

Then perhaps as much it should be explained in the feat? 

Quote from Nitro2go >>

Then perhaps as much it should be explained in the feat? 

 You mean like this?:

When you choose this feat, you acquire a ritual book holding two 1st-level spells of your choice.

If you come across a spell in written form, such as a magical spell scroll or a wizard's spellbook, you might be able to add it to your ritual book.