Hi Nancy, this is Kimberly from Landscape Solutions For You. Thank you very much for asking the question on your Gardenia plants. You live in the same climate zone as I do, zone seven. Gardenia's can be tempermental in our zone. The yellow leaves could mean a couple of things. We have gotten some good rain, and sometimes when we get a large amount of rain, some plants that are getting situated in their growth patterns will have their leaves turn yellow. I would recommend to test the health of the two Gardenia's that aren't doing so well. Scratch the surface of a branch and see if it is green or brown. If it is green, the plant is alive. If it is brown, it may be in shock. Either way I would loosen the soil around the plants and put some organic mushroom compost around the plant. This is an organic fertilizer that will last up to one year. It doesn't smell great, but your Gardenia's will love it!
If you think the two plants are possibly dying, depending on how long ago you purchased them, see if their is a warranty on the plants. You could bring a branch in where you bought the plant and see what they say.
I am not sure what variety of Gardenia you have. There are deciduous and evergreen varieties. Let us know how the Gardenia's branch looks, and also keep us informed on how they are doing.
Thanks again Nancy for asking the question and feel free to ask a question anytime. I hope this little bit of information helps. Best of luck to you with your Gardenia's.
Kimberly