It sounds unnecessarily complicated. I would simply make a berm and plant in that. Choisya is not a fussy plant. Al

Since choisya originates from Mexico, they are well able to withstand summer heat although IME gold foliaged forms tend to scorch a lot in direct summer sun. I would not recommend attempting to keep this shrub to less than half its typical size - the constant pruning necessary will pretty much eliminate any flowering potential, a primary reason for growing this plant. To my knowledge there are no dwarf forms, although the hybrid 'Aztec Pearl' does not get as large as the species ternata. But I'd still expect 4-6' feet and relatively quickly at that.

Are all three in one container? This is potentially a 6ft x 6ft shrub so they are possibly just starting to get hungry in cramped conditions. Is there a reason you need to have them in containers? If so I'd give each one its own pot as large as you can find and afford. If not they'd be far happier in the ground where watering and feeding can be much more free and easy. Here is a link that might be useful: Aztec Pearl

Most plantings of this commonly used shrub don't get any coddling. If concerned about winter cold be sure ground around it well mulched, to retard freezing of the soil a bit. Maintaining a mulch on plantings improves plant response in general, should therefore be done anyway. If temperatures below 10-15 degrees F. threaten you could then rig up a screen around your plant, for additional assurance. Something insulating, like a tube of wire mesh fencing held in place with stakes, and stuffed with loose, more or less dry plant litter (maybe small conifer branches) to reduce wind and sun exposure during the worst cold. Or maybe a cover made out of synthetic insulation material, like styrofoam. Take it away as soon as the Arctic front breaks up.