Q. I planted about 12 gardenias a month ago and if them appear to be dying. I checked the soil and it's evenly damp. It has two 2gph emitters and they are all being watered 3x a week for 20 minutes right now.
A. From the picture it looks like a lack of water. Just before you tell me "no", I want to explain something. A lack of water to the plant can be for many reasons and only one of those reasons is that not enough water is being applied. Follow me on this.
Water is applied to the soil. The water fills all the spaces between soil particles forcing the air which is in the soil to exit. As this water drains or is being used by the plant, outside air above the soil begins to enter into the soil filling the spaces that was previously filled with water.
Roots of plants need oxygen. The above ground, green portions of the plant need carbon dioxide and oxygen. If the soil particles are too close together there will not be enough room to provide the oxygen needed by the roots. This is why we add compost and other organic material to the soil so that we can build the structure of the soil to provide that oxygen needed by the roots. I know you have improved that soil for those gardenias so that is not the problem.
There is another problem that can occur which can keep water from moving from the soil to the roots and finally to the leaves and stems. If there is something which is choking the stem of the plant, this can prevent water moving from the roots to the leaves. Mechanical injury to the stem from insects which chew or bore holes into the central stem can disrupt water moving from the roots to the top.
There is also a disease problem which can choke the stem near the soil level. This can happen on very young plants which have stems surrounded by wet mulch. If this mulch stays wet around these young stems, it can cause a rotting to occur at the soil to the level of the mulch. This is frequently called collar rot. Make sure that wet mulch does not come in contact with the trunk or stems of young plants. Young plants are very susceptible to this problem. As they get older and developed more wood around the trunk or stems, this problem lessens.
So from judging just from the pictures to me it looks like water related and/or too much shade. Excessive shade can cause thin, spindly growth that can't support itself. That is my best shot at it from the pictures.