Most of my balcony tomatoes have been doing well, but this plant started to seriously wither 2-3 days ago. It is about 3 months old. It is well watered. Container has drainage holes. Fed with liquid nutrients about 2 weeks ago. Weather has been 26-28 C, with mild winds. South side balcony, so getting quite decent sun.
First check if water is reaching the roots by poking a finger or a stick a few inches down into the pot- the soil should be moist all the way through, not just a thin layer at the top of the pot.
If soil moisture looks good, it could be fusarium wilt or verticillium wilt, though your photos don't indicate the tell-tale leaf yellowing of those diseases.
From Kansas State Research & Extension: "Verticillium wilt tends to develop during the cooler periods of late spring. Symptoms of this disease are similar to Fusarium wilt. Older leaves are affected first; they turn yellow, wilt and eventually drop from the plant. Unlike Fusarium wilt, Verticillium wilt causes uniform yellowing and wilting of the lower leaves. As the disease progresses, younger leaves begin to wilt and die, until only a few healthy leaves remain at the top of the plant. "
One more thing to check is to look at the base of the plant, and make sure that there is no damage to the main stalk. Damage to the stalk can limit water uptake. Causes of damage could include chewing by insects or tearing due to wind, rough handling, or heavy branches that are not supported.
They are looking similar to my tomato plants and I had the same issue (Why are my tomato plant leaves curling).
It has been suggested that they need more water so that could be your issue.