The OTO tester can also be used to measure bromine and thus has a bromine column on it. I'm assuming that when you say that your chlorine is 3-6, you mean 3 and the 6 is in the bromine column. (Just ignore that.)

Go ahead and add some more bleach to get back up to about 6ppm --- assuming that I am right in my assumption above.

Yes to your question about another dose of Borax and baking soda.

No need to test calcium hardness. CH is only an issue in a vinyl or soft-sided pool if it is too high. Yours is fine.

Do you have an HTH 6-way drops kit or an OTO kit? Just wondering how you got your CYA reading.