aww, poor little guy, guess he's pretty confused now. Ask stbx if s3 is having accidents at his place, if he is, then it is just a matter of potty training him again, or perhaps stbx isn't taking him to the bathroom before bedtime or when it's needed, calmly bring up this concern to him so he can try to do a better job to help s3 go to the bathroom often.
I half remember when my d5 was 3 she did try a few times to have us hug or kiss, perhaps when he does this pick him up and kiss him and tell him you love him and that dad loves him too.
The meltdows are somewhat typical, but you are right, you can't keep calling his dad when he does this or he'll use it as a weapon more often. Just calmly tell him that he isnt' here and that mom is here to help him, be consisten and don't call his dad anymore. 3 is a hard age as they try to find who they are and kids are trying to find their voice.
Be not afraid...I will repay you for the years the locusts have eaten Joel2
30something 2kids survivor of S, MLC, A, D I have peace in my heart, at last.