What is this disease? It randomly showed up rather quickly on three tomato plants
One more photo. The first was of the bottom last leaf. This one is of the top of the plant.
Many tomato diseases are regional, so I'm not going to comment until you share with us , just roughly, in a geographic sense where you garden and a gardening zone would sure help.
I first checked Your Page for your location since none was given next to your user name, but no info was there either.
Carolyn, going on an assumption of possible S CA coastal.
Looks like late Blight.