I would say either 1 tomato plant by itself or 2 bell pepper plants by themselves in there. There's not enough room for 2 tomatoes and 1 bell pepper in there.

Well, it's the smallest raised garden I've ever seen, but I would put two tomato plants in it. :) If the tomatoes grow well, I'd expect the pepper plant in the center to be shaded and crowded.... but then you will also have the choice of removing lower leaves and suckers on the tomato plant to help with that. The additional bean plants will have even less space and light. I hope they're just bush-style and not climbers. :) I assume the beans haven't been planted long, so they could be easily lifted and moved elsewhere.

Re yellowing leaves, I don't see it in the photos. Lots of rain and lack of sun won't help. What soil/mix did you use to fill your raised bed, and have you given the plants any fertilizer?