Lookup with comparing values ?

I have a table with two columns. The second column has numbers.

I am trying to do a lookup that will check to see which number after my lookup value is +3 bigger.

example

a 11
b 5
c 5
d 6
e 10
f 9

I am looking up "b". I want to return "e" because (a) it is under b in the sequence and (b) 10-5>=3. Is there a way to do this with lookup? I checked the forum but couldn't find anything about lookup and difference in values.

Thank you in advance.

Kostas