by dual recto i think you mean 2channel recto, since you mentioned 3ch dual recto later on.
To break it down:
Dual Rectos:
2channels: less features, clearer tone (note separation)
3channels: more features, bit harsher tone
Triple Rectos:
these come in the same as above, 2 or 3 channels.
The only difference is a triple recto has 150watts instead of only 100. You really do not need the extra 50 unless you are metallica and will be playing donnington. 50 watts alone is enough to break a window (literally).
3channels were introduced in 2001 or so. 2channels are only available used (try ebay, craigslist..)