Ok, if you are 100 percent sure that no-one could be touching her there, my question is have you tried changing washing powder, soap, toilet paper, disinfectants etc, anything that could come in contact with that area. Even if it's not irritating her anywhere else, that area may just be extra sensitive.

Try stuff for sensitive skin (but note, both my children have such sensitive skin that they're even allergic to some of the special prescription stuff for sensitive skin) you may have to try several before finding what works.

But don't change everything at once. Start with the toilet paper & see what happens, then the soap etc. Also you might get a gentle cream(savlon? Something you have used before on sores or in that area to just help ease the irritation). One other thought just hit, many people I know, including myself have found that a warm bath with a couple of cups of apple cider vinegar which you flick/wash like waves into that area has helped rid irritations. Cheap & easy to try, but you need to do it for several days & with a 3 year old i'd suggest twice a day.

Also make sure she's not playing in sand or dirt, as this could work into her pants (even just abit) & could be the cause of the problem.

Good luck.

It sounds like your daughter might be allergic to soap. Alot of people including children are vert sensitive try letting hertake a shower.Make sure she is wiping from front to back.If this doesn't work try using blistex or vagisil.My freind does this an it works for you.
Good luck

Did you put any cream around your dughters vagina try useing baby soap for a while to see if that helps out any I feel her pain.When she pees does it burn her at all in the vagina make sure she uses a warm compress twice a day tht will help her at night time please write me back , if that work thank you.