I just wanted to say, that I think that pregnancy hormones have alot to do with the feelings of panic even on a third. The feelings and emotions and thoughts you are having right now are normal.
My dh and I planned our third, but still had all the thoughts you are having. Despite lack of permanent jobs for both of us, we were just ready to go for a third, I got pregnant and miscarried early, but that made me more determined to be pregnant again, and in less than 4 weeks I was.
I was excited at the prospect of a third, that was until I was pregnant again, then during that pregnancy was worried about everything, "how will I cope with 3", "what if there is something wrong", "what if my dh does not get another job", "what about my work, will I be able to work again with 3 children at home?"..... the questions went round and round my head and I regretted getting pregnant. However..... My now 3 year old third child is one of the greatest blessings in our lives. He brings so much love and joy to our lives, I can hardly believe that I was worried about so much when pregnant with him. My worries stayed with me through out my pregnancy unfortunately, but vanished within a couple of days of giving birth. I hope that for you the worries leave earlier than that, and that you can enjoy your pregnancy.
~please be reassured that your feelings of worry are normal. I hope that sharing here has helped and finding others in a similar situation is comforting.