To what extent do the levels of UVA, UVB and UVC radiations on the earth surface increase during solar flares?
During a powerful solar flare the amount of x-ray radiation can increase up to 110% in the boundry layer of the atmosphere/space,very little of this will reach the earths surface but it can be measured from high flying planes.
UVC is stopped by the upper layer of the atmosphere so this will never reach the ground,It might be possible to measure some UVC in antarctica because of the ozone hole but it is very weak so one will need special equipment to measure it.
UVB will change very little during a solar flare and only in the shortest wavelenght of the UVB spectrum will additional radiation be measurable.
UVA radiation will not change at ground level.
During a solar flare only the shorter wavelenghts of the electromagnetic spectrum will be affected,hard x-rays the most and the longer wavelenghts will be less affected.
source for more detailed information https://en.wikipedia.org/wiki/Ultraviolet
i do not have the source for this picture but i think it is free to use.
Solar flares are very variable in size and position. A small one on the edge of the sun's disc would have no effect on Earth, but I presume you mean a large one. UVC would be stopped by the ozone layer, so no increase there. UVB gets through the atmosphere with difficulty, so there might be a small increase there, but nothing to worry about. The atmosphere is completely transparent to UVA, so there could be a significant increase with a very large flare. You wouldn't know this unless you followed the TV weather announcements of ultra-violet hazard. If it happened to be cold and wet in your part of the world it wouldn't affect you unless you were in the habit of sunbathing in single-figure temperatures, lowering cloud and pouring rain. Not many people do this.
More worrying for most people would be the flare's effect on satellites and communications, or even to the electricity grid in the event of a really large coronal mass ejection. The arrival of huge amounts of charged particles emitted by a flare creates electric currents in the atmosphere which in the most severe cases have been known to shut down electricity grids for many hours.