What should we do regarding Foster Children?

Ok my husband and I have adopted our daughter, she is now 10 mts old. 3 months ago our local dhs contacted us and stated that her birthmoms 2 other children had been placed in foster care and wanted us to take them until their mom could meet her program. We agreed to foster the children. They are 3 and 18 mts old. It has been 2 months and i am afraid we bit off more then we can chew. I feel like our daughter is not getting the attention she deserves because the other 2 children need a lot of attention due to being neglected etc. I feel awful that we are even contemplating not fostering them anymore, but I dont know what to do. My husband and I are so stressed out and tired that i dont feel like we are being all we could be to any of the kids. Do we let dhs place them somewhere else? or do we continue to try and raise the kids at the expense of our family? I feel like if we let the girls get placed with some other family that is will be bad for them cause they are attached . The birthmom still has to finish 2 or 3 cles plus get her ged and get housing so there is no telling how long the kids will be in foster care. Any answers appreciated...