My girl is 43 days post mating and has been scanned to confirm pups.
I am currently feeding her RC starter with some fish but she seems to be hungry all the time. I keep reading how it's really bad to over feed a bitch but despite getting over the recommended amount and feeding her 3 times a day she is still a bit hungry.
Should I increase her food so she isn't hungry at all?

I'm all for not allowing a pregnant bitch to put on more weight then she needs - it's far better from a whelping point of view, for her to be lean rather than FAT.  Just as it is with humans really.   Is she normally greedy because she may just be 'cashing in' on her condition right now, knowing that you are probably going to (over)indulge her!!    Fact is once the puppies arrive, and you see exactly how many (scans are not accurate and only confirm a pregnancy at the time of the scan) she has to cater for, then you pile in the food.   As long as she's having her normal good quality food, I see no reason to give her that much more than usual.   The puppies will take what they need from her, both before and after birth.   And it's really only into the second week, and up to when weaning begins that she'll need loads, especially if she has a big litter.    Some bitches, despite being fed loads, will give their all to their litters and come off them being quite badly out of condition.   Not to worry - that will come back fast, once the puppies are making no demands on her.

You will probably have to split her daily amount into several small meals as she gets bigger and the puppies start taking up more and more room inside.   And then you also cut back on the bulk, concentrating on protein.

But caution - encouraging too much lactation too early could lead to mastitis.   Newborn puppies don't actually need, or take that much milk - again it's into the second week that it all kicks off.   Also I'm no fan of Royal Canin other than maybe for small dogs.   For mine, and other largel breeds, the protein count is too high.   I've seen the result of 'too much, too fast' with this food.

Having twice had bitches that lost around 50 % of their body weight after their first litters were born (they looked like RSPCA cases), I now feed as much as they want from week 6. But then I have breeds known to give birth easily.

Same as Marianne.  I feed to appetite, they get what they want.  My bitches have actually put on weight by the time pups are weaned.  But I couldn't do with a bitch looking like a neglected rescue.  I also like pups slightly podgy.  If they became ill, with my breed, they would be hard pressed to deal with any weight loss.  I find that puppy fat is soon lost as they become more active. 

I do feed a high protein kibble with any meat the bitch fancies.  I  try poached white fish, minced steak etc.  I do find that they go off kibble once whelps are being continually cleaned by mum.  They do tend to get sore mouths.