You are going to have to set up a quarantine tank for all 3 fish. You can either cure them with either hyposalinity treatment or copper.
Both these treatments will kills any inverts in your tank so you must set up a quarantine tank for them. There are hundreds of other things to try, but if you want to truly rid your system of ich, use hypo or copper treatments.
While you are treating your fish, your main tank must be left with no fish for at least 6 weeks so any remaining parasites will die off by not having a host fish to live on.
The pits on the tangs face may be bacterial or the start of head and lateral line erosion. Both are different treatments than ich.
I would bump up your SG
a bit. Natural sea water is 35ppt which is about 1.026
Temp can go up a degree or two too.
What kind of test kit are you using for amnonia? Some are really hard to read.