All are planted in a raised garden box with a mixture of topsoil and potting soil. I have two plants that are less then 3' away and they are doing great.
My Tomato Leaves on One Plant Are Shriveled, but Not on Surrounding Plants
Certified GKH Gardening Expert
There are several reasons this can happen, from disease to root damage to fertilizer burn. As the plant is failing, I would recommend removing it ASAP. If it is failing due to disease, that disease could spread to nearby tomato plants if it is left in the ground.