On average babies usually get bigger as you have more, but not always.
My DD1 was 7lb 5oz, my DD2 was 8lb 9oz & my DS was 8lb 7.5oz.
So my 3rd was slightly lighter than my first (all have the same dad, but I don't think that'd come into it too much)
It is likely that your 3rd will be a bit heavier, but it shouldn't be a problem