Well, it's only the 3rd date, afterall! Maybe he just wants a lady friend for company. And maybe she's a nice women, and not someone looking for a sugar daddy with one foot in the grave and the other on a banana peel.
I'd at least try to meet the woman and get a feel for how the relationship is going before saying anything to your dad. Once you have a little more info to go on, you can decide when, if, and how to bring it up.
Seperate from his dating, someone else sugested you find out if he has a will or trust anyway-I agree and second that thought. You don't want to find out after the funeral that he left it all to the squirel in the back yard or something. Worse, if he has no will, then the estate has to go through probate and the lawyers will get it all, plus it could take months or years to sort out.
My parents (both alive and married to each other, thank goodness!) sat down with my two brothers and I and told us what they'd done. They have a living trust, so upon their deaths their estate is split equally between us, and the trust is in our names, not "our children" so some one else can't claim to be a child from an affair or something (I guess that happens sometimes, people read of deaths in obits and get just enough info to try to claim some stranger is their real parent and try to get in on the inheritance. People make me sick sometimes.) Even if one dies and the other re-marries, the survivor has to actually change the trust to include the new spouse, it would not be automatic that the new spouse received any pre-marital assests.
Also, my parents have nursing home insurance. This covers a lot of options if one or both need either in home assistance, an assisted living home, or a full nursing home. It pays for almost everything, so it won't drain their retirement savings. It turned into a blessing, for a few years after signing up for it, my father was diagnosed with Parkinson's. When the time comes that my mom can't take care of my dad by herself, she won't go bankrupt trying to get in-home nursing help or paying for a home. (Right now he's fine, the medication is working wonders!).
As touchy a subject as this is, you need to at least find out if there IS a will. How he decides who and how much he's leaving his estate to is his buisness, but he should at least let you know if one exisists and where all the important papers a located so you don't have to serch for them upon his passing.