The answer is definitely EYE.
Explanation:
1. Pronounced as one letter...(pronounced as "I")
2. And written with three,....("EYE" has 3 letters)
3. Two letters there are......(two alphabets used are "E" & "Y")
4. And two only in me....(Probably Ravi has 2 eyes)
5. I'm double, I'm single..(Few have 2 eyes & few others only 1)
6. I'm black, blue, and gray..(That's true, we can see people with black or blue or gray and probably brown eyes)
7. I'm read from both ends
And the same either way.....(That's obvious!)
What am I?