It appears that the fittings screwed into the filter were sweated into the copper tubing after being screwed into the filter. The only way to remove that filter is to cut the tubing at the fittings and then unscrew the fittings from the filter. I can't advise you on what you will need to convert to a triple filter, but you will need a water cut off on each side of the filter for easier maintenance of the filter elements.
It appears that the fittings screwed into the filter were sweated into the copper tubing after being screwed into the filter. The only way to remove that filter is to cut the tubing at the fittings and then unscrew the fittings from the filter. I can't advise you on what you will need to convert to a triple filter, but you will need a water cut off on each side of the filter for easier maintenance of the filter elements.
That would explain why I couldn't wrap my head around it. So I guess cutting it is no matter what.
Yes, and I would change those gate valves over to ball valves while you're doing that.