Waterfall,
If you have not tried the selenicereus megalanthus, then I would hold off planting this out. Many people, myself included, find the taste odd and generally not nice. If yours is the mega x White hylocereus, it will taste exactly the same as the white variety--which is to say, very bland. The yellow mega is also not a very good pollinator--at all.
The purples are the best, followed closely by the reds, which are trailed quite a way by the whites. I have one of the largest collections in Australia now, and can sell you some better varieties if you wish.
Your trellis system will work--but be careful with that treated pine. Because dragonfruit are epiphytes (kinda) they will absorb the nasty chemicals in the wood. I am currently moving into farming them, and have recently learned that you cannot get organic certified if they are grown on treated pine trellises. Those pots are a great size, but I think you should elevate them to stop them pooling water; standing water will absolutely kill your plants. If you don't want to do this, lay down a few inches of gravel at the bottom of each pot. Crossbeams joining the 'roof' will save you a lot pain down the track; young, heavy shoots have a tendency to snap unless supported. This can happen without you noticing--these partially attached limbs will not appear dead, but will not grow.
I recommend planting out both your reds and the white for pollination. Ditch the yellow. There's more than enough space for three plants.
