I'm on second coat, some seams look like the tape dried away from wall. SHould I sand it down and apply more tape or just mud over w/ a third coat then sand?

I suggest cutting out the bubble with a sharp knife - cut through the tape and remove the portion that isn't sticking.to the wall board. Then spackle over and sand.

Thank you!

Sometimes you can get some air trapped behind the tape, also if the crack or seam is larger than normal this can also happen, but yes cutting it out will work unless you are needing to build the mud up some more, keep in mind if you cut it out a 3rd coat want be enough to patch it fully, it will need an extra coat on that spot