I don't recall what the differences between 3.0 and QuickBASIC are, though I believe that QB is newer. QB is the one which I used on my SE/30 back in the day. 3.0 has separate versions for decimal and integer.

As for .hqx, that is an ASCII file encoding, so you'll need to decode that with BinHex or StuffIt and then there should be a .img file within. They are copies of my own disks if they had the orange star on the download page.

As for command list, I never had a manual for either 3.0 or QB, so I had to look at existing programs to learn the commands back then. Just recently, however, I was given the manual for QB, so I can make a command reference for serial ports if you would like.

QB has an Examples disk which has a Terminal program, so you can also look at that.