I'm not a boogie owner but I know a fair deal about amps.
Recto refers to the rectifier section in the amp (making DC out of AC)
before ca1966 almost all amps had tube rectifiers (as well as tube pre-amps and tube poweramps) after that, due to demand for greater power (Hendrix, the Who) amps started to get equipped with solid state rectifiers.
Tube rectifiers suffer from a lot of sag and compression.
Mesa tought it was cool to offer both, so they came out with a
Dual Rectifier amp: switchable from solid state to tube rectification.
Single Recto and Triple recto is just marketing BS.
- The 'single' recto uses solid state rectification (like ~99% of amps, tube or not)
- The Triple recto is a three channel dual rectifier (like the regular dual rectifier, which also has three pre-amp channels) the main difference is power, the triple is 150W (the dual 'only' 100W

)