It is worth noting that the difference between 3/4 and 6/8 in this model is that a 3/4 measure consists of 3 beats each made up of 2 sub-beats and a 6/8 measure consists of 2 beats each made up of 3 sub-beats.
Hence simple vs compound time is distinguished by 2 or 3 sub-beats per beat.
Note that the notion of a beat group is degenerate in this case and is only useful in cases were the number of beats per measure is more than three.