Your pots are big enough for dahlias. Hawaii is 4 ft tall for me, so a stake would be recommended when you plant it, or plant it next to some kind of support- fence, deck uprights, etc. I hated the crosswires in the pot and found them unnecessary.

Don't know cactus soil mix. It sounds sandy. In pots in a sunny location, one enemy will be moisture loss. I used Soil Moistening polymers in 1/3 compost, 1/3 topsoil (with more compost in it) and 1/3 potting mix. Still, on the hottest days in black pots they needed water daily, once up and growing.

They should grow well and the tubers do nearly as well in a pot as in the garden. On the taller (4ft) varieties the foliage was more sparse, but the flowers were fine. I might have missed topping those, though. 3 footers are great in pots.

My opinion would be to only put one Hawaii in each pot rather than crowd the tubers. Three would be too many, two would look funny. How tall is 'Yellow Eye?' If it's shorter, maybe you could do one Hawaii and two yellow eye in a triangle.

Do a search on growing dahlias in pots and you'll find more links that may help.

Here is a link that might be useful: Growing Dahlias in Pots from ADS site