While I respect the fact that you are really trying to think about your immediate family's needs and your mental health and the impact of everything on everyone, I would urge you to find someone (professional) that you can talk to about all of the issues that have been brought to this thread.
Your own MH/depression needs will be sorely tested by any outcome of this latest pregnancy - whether or not you keep the baby - you will likely be put through the mill as you are between a huge rock and a hard place - it seems to me anyway.
The needs of your partner and especially your two little ones need to be considered very seriously for the reasons outlined so well by ethanchristopher and onestonetogo.
This will potentially have a massive ripple effect on all the people around - not to mention people outside your intimate circle of friends and family who will ask you about your pregnancy and afterwards will ask about the baby - you will need some help to know what/how much to share with close frineds and strangers alike. You will need help to decide upon your 'cover story' - so that you don't need to explain or justify your decision to all and sundry, but you need to be comfortable with what this is and how you share it.
Please do also consider the implications of this decisions upon your unborn baby. Thank you for considering adoption over abortion as your choice, but as a parent of an adopted DD I want you to know that this decision will (potentially) affect this baby very much. A great many adopted children - even those who have not encountered huge suffering - live with a sense of displacement or rejection. Your decision will eventually form the basis of this baby's life story and his or her sense of who they are and where they have come from. Adoption may seem like your only option, but it's certainly not the easy way out, for anyone.
Your decision is MASSIVE for you and for the baby. Please get some proper professional counselling help to work through all the issues and decisions that are facing you right now. Much respect to you for so trying to do the right thing - I don't have the answers, but would urge you to get appropriate help.