If memory serves me right with ACT! 3,4,5 & 6 you need to use OLE to connect to the ACT! system and consume the SDK.
The ACT! classic dB system had several flat file "tables" in dbf format e.g. the main contacts dB e.g. actdemo.dbf but that would only give you access to field level data.
To access the Notes/History etc you would need to interegate the *.blb file (BLOB file). You could do this if you knew the header settings for the blb file (I dont sorry to say!) to make head or tail of the binary data, therefore the easiest way was to use OLE.
What this would mean I suppose is that if you use VBA you should be able to get to the ACT! 3 system.
I know that is not much of a help but I don't have my old ACT! classic SDK manuals anymore.