for a single-speed conversion, is it always necesary to change bottom bracket? I want to convert my old raleigh, which has 3 chainrings on the front, into a single-speed. Do I have to replace the bottom bracket? I keep hearing conflicting things. 
Thankyou.
You need one of four things:


An eccentric bottom bracket. Like these
An eccentric hub. Like these
Horizontal dropouts. Like these
A chain tensioner. There are a number of different types.


You only need one of the four, but these are the only ways to get the chain tension correct on a single speed. (I should say "almost only." Blind luck works every so often.) Additionally, as others have mentioned, a chain tensioner will not work with a fixie.  

It's also possible that your spindle is too long or too short to get your chainline correct. If that's the case, you'll have to get a new bottom bracket with the correct spindle length even if you already have an eccentric hub or horizontal dropouts. It's usually possible to position the chain rings on the inside or the outside of the crank's spider and get it correct, but not always.