My 2 yr old have high fever for three days and a cold what could this be and what should i do? his lil sister has the fever also.

Watch closely: It sounds like something contagious. Possibly viral given the presence of "cold". Runny nose, cough, mild to moderate fever are suggestive of it. Flu is another option - higher fever, less upper respiratory symptoms, except for cough, more ill appearing. If no vomiting present, try tylenol/ibuprofen, cool sponge baths to reduce the fever. If no other symptoms, have him take fluids and watch close.

Time to see the doc: I would probably get this checked out. This may represent a secondary infection such as an ear infection or pneumonia. This may also be a continuation of a viral infection as well.