Estimates based on individual Swim/Bke/Run times

I've not yet done a tri, but cycle much and have ran road races a lot. In the running world, there are fairly reliable ways of estimating times across distances based upon your time from a single race distance. Is there anything for tri's that can estimate your times based on individual sports?
I just have no clue as to what kind of time loss I should expect in my run form the other preceeding events, similarly how much I should slow down on the bike etc. Do you just jump into a tri and see or can you get a good starting estimate?