I agree with you, I think...Splitting things three ways doesn't ensure that the kids are all "treated equally", as there is a disparity to begin with.
The two other children will have whatever comes their way from their other parent. Which includes the assets already given up by your DH (potentially, of course, as you can't be sure what their mum will do - she might leave it all to a home for cats!) . If you treat your own and DH's assets as joint, and then divide them by three, the two older children will receive assets from you, from DH, and via their own mother. DD will receive assets from you and DH.
I would think that DH's assets (not that he has any, it sounds) should be distributed between all three of his children, whereas you are perfectly within reason to want your assets (especially as held prior to your marriage) to go to your own DD.
To what extent do you normally combine your finances?
DH and I haven't made up wills (very bad, I know, and we were JUST talking about it the other day!) - but like you, I have one DD with DH, and he has two older children from a previous marriage (their mum is deceased though, and they live with us fulltime). We don't have any money or assets to speak of, so it is pretty hypothetical...But I would ideally want to ensure all three kids ended up on a sort of 'even playing field' - the older kids have money put aside towards their higher education, for instance, whereas we have nothing set aside for DD. So I'd certainly take that into consideration when planning. The older kids will be independent adults well before DD - so if something happened to me or DH in the next few years, it would affect DD much more deeply and for longer (in financial terms, I mean!) - so I would also take that into account, as being able to support a child would take priority over an inheritance to young adults, IYSWIM.
you decided to go into a relationship with a man who had children -it's all about compromise
pish posh.
You and your DD are already disadvantaged because of his financial choices to date. That is a separate issue, to some degree, but I think in your shoes I would be concerned about protecting DD's future, as the other kids seem to be well provided for already.