Hello N.,
Thank you so much for posting this question. I haven't encountered this issue as of yet but fully expect that I will. Like you, my son's father and I separated before he was born. My son is now two. His father, while publicly professes that he wants to be an active father; has never assumed that role. I can count the number of times he has visited.
My son hasn't yet begun asking for his father but I fully expect that he will. I had assumed that I wouldn't have to deal with this issue for a couple of years yet but based on your experience perhaps this is coming sooner than I expected.
If my son were to begin asking for his father now, my response to him would be,” your daddy's not here (or can't be here, your choice) but mommy's here and will always be here. Mommy loves you”. At the tender age of two (or in your case three) I would think that response, coupled with hugs and kisses, would suffice. If you have another male in your life, be it your father, brother or a best AND trusted friend, for the time that your daughter is so young and asking for a daddy, redirect her "father-need" to another family member. For instance, if your father is actively involved in your life, each time that your daughter asked for her daddy, respond by saying something to the effect of , "honey, no daddy here but grandpa (or poppy) is here and loves you very much. Do you want to talk to grandpa?" If she says yes, then call your dad and let her speak to him. Be sure to speak with your father or whomever you selected to play this role to gain their agreement and support in advance of supplying this response to your daughter.
I hope this helps. Good luck with your daughter.
E.