What can you see when the third eye opens?
The energetic outer world to some extent; mostly your inner world i.e.your subtle body, chakras, energy moving as the elements or light. Your intuition is enhanced.
Sey 
Hi Tomlle:
See Lesson 416 - “From Third Eye to Sixth Sense and Beyond”.
Also see other lessons on the “Ajna” (third eye) in the topic index.
All the best!
The guru is in you.
Yogani and SeySorciere,
Thanks to both of you.
HI SeySorcerie & Yogani I see mostly brilliant bright yellow white light is this a stage on the way to seeing more?
So far unless something release up from the subconscious usually felt coming out of a lower chakra there is nothing else and I can always tell by the rising up feeling when something is being released and then it takes a visual form other than that just the glow…
Hi So-Hi:
More like on the way to seeing less (dimensionally) and intuiting more (non-dimensionally). 
Good things are happening.
The guru is in you.
Dear So-Hi,
I don’t know how these things work. In my case it started with a wish / an intent to see. I remember complaining to Yogani about how come I couldn’t see any light. Shortly after, I started seeing. First thing I remember seeing was my heart shining bright rays of light.
Sey