Welcome. My name is ***** ***** would be glad to assist.
Looking at your information, you posted this:
"one of the neutral wires is grouped with the hot (red) and the others are separate. I failed to mark them when I removed the previous light fixture and do not know if the other three were all together"
Based on that, the 3 black neutrals will tie together.
The one black connected to the Red Live wires is the down feed to the switch.
Then the red from the switch is the Live feed back to the light.
This is done, because the same cables are used for switches and so you have to use the black as a Live and not a a neutral. Neutral does not connect to a standard switch as in your situation.
Look at the diagram below, it may help
Let me know anything further,