Always tell your little girl the truth - dont cover up things to protect her - tell her in language that she understands - but tell her the truth. You may have loved him, but her father walked out on you both when you needed him most - that is not slagging him off - that is the truth.
Saying that you loved him very much and that he got scared may paint quite a rosy picture for your daughter. The truth is he walked away from his 3 month old daughter and never looked back - hard as it is to accept - he is unreliable to say the least and this is the truth - your daughter must try to understand this - and you dont want her upset again - you are worried about her because you care about her - tell her this.
Talk to your partner first and get his support. You have two choices - help your daughter to contact him now or tell her that you feel she is too young at the moment, that you are worried about her getting hurt but that in a few years you will help her to contact him. Promise her this, and that may well be enough for now.
If you decide that now is the right time then tell your daughter that you know where "biological" dad lives and that a lot of time has gone by and he is married now and that you dont know how he will feel about contact with her. Explain to her, that if he doesnt want contact now, he may decide he wants contact in the future, so not to get her hopes up. None of you know how he will feel, so the best bet is a joint letter to him, you write it, let your daughter see it, read it and help you with it. Then wait and see what happens.
Explain to your little girl that her "real" Dad is the one that is looking after her every day, he loves her, and she does need to talk to him about it too.
Good luck - terrible thing to leave and never return - one day when she is older she will realize just how terrible that is - and it is you and her "present" Dad that have