Rh neg Mum Rh pos Dad Potential Rh pos babies. What were your first 2? If they were Rh- then you have nothing to worry about at all.
if you have had no problems, i.e. if you had no foetal blood leakage into your system at any point during pg and delivery and you didn't need any extra anti-D injections, then there is no extra risk.
Risk comes from Rh pos babies - the first one is generally not an issus because any Rh+ blood will be the mum's first exposure to it and antibodies take a long time to be made, plus they are IgM antibodies which are too big to cross the placenta. IF the mum makes antibodies with a first baby (or a 2nd baby), then any subsequent babies could be at risk.
If you had all your anti-D shots in your 1st and 2nd pg, and no traumas, then it is very unlikely that you have made any anti-D and so your 3rd baby is not at any greater risk.
When your baby is born, they take a sample of its blood from the umbilical cord - this is checked for Rh status. If it is Rh+, then you get a shot of anti-D. If it is Rh-, then you don't. If you need the anti-D, a blood sample should also be taken from you for something called a Kleihauer test - this is to check if there are any circulating foetal cells in the maternal blood stream. If there ARE, then more anti-D is given.
The system generally works very well - there are a few exceptions but if you had any problems before, details should be in your notes. Have you had your booking bloods done? Rh- pg women always have their blood checked to see if it contains any homegrown anti-D and if it does, then more checks will need to be done on your baby during pg. If you are clear, then it shouldn't be any different from your previous pg.