Lese
The tablets you are giving your daughter are anti-histamines, which are good for relieving the symptoms of rashes like itching. You must stick to the maximum dose, and do not give her other anti-histamines as well, as this is not safe to do.
The question has to be what is this rash? Three years to have a rash is a long time! Has her doctor told you what it is? A small nettle like rash sounds like an allergy of some sort.
Has she eaten/used anything over the weekend that could give you a clue as to what is causing this?
If it's eczema you could try emollients, like aqueous cream (rub in gently or use as a soap substitute) or bath products like Oilatum - both can be bought from the pharmacy.
Try a look at this website.
http://www.freederm.com/