How do you know if your ready for a third baby?

Maybe I sound crazy but even though sometimes I wonder how I handle my kids now I still can't stop thinking about having another baby. My fiance & I are only 27 & we already have a 3 & 1 yr old daughter. Idrk if we'd be able to afford it but that doesn't stop other people lol. It's just crazy to think that if we did it now my 1st daughter would already be 4 yrs apart from the new baby. We dont really have any family help so its just us all the time with the kids. Id like to make my own family with my kids since we dont really have family. What do you guys think? What helped you guys decide when you were ready for more?