Legendary engrams, are they biased towards the character class I'm decoding it on?

When I decode a legendary engram, and it produces a legendary item, sometimes it's for a different class.

Is this truly random, so that my warlock has just as large chance of getting a legendary as the two other classes, or will the legendary most of the time be for my class?

They are not biased towards your current character, legendary and exotic armour engram rewards are based on RNG (Random Number Generation) and so are completely random. For example the exotic engrams that Xur sells from time to time can give you an item that could be for any of the 3 classes.

See this image for an example of a legendary engram, saying it could be for (Random Class): http://i.ytimg.com/vi/vbw8GIeOx3A/maxresdefault.jpg

However if you get an item for a different class, it's not the end of the world. When you decide to start a second character, or 'alt', you'll have nice equipment waiting for them! Either that, or you can just dismantle for some ascendant materials.