Answer: If only the lower leaves of your plants are yellowing, it could simply be a natural process. Plants tend to drop their oldest leaves in favor of growing new leaves and flowers. Since they have been in the ground for 3 weeks, you've probably seen the worst of the transplant stress (which includes yellowing and dropping foliage as the roots become established in their new homes). So, the yellowing leaves might be the tail-end of the stress. I'd continue to water deeply once or twice a week and pick off or snip off the yellowing leaves to make the plants look more attractive.
Best wishes with your new garden!
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