My three year old son has been sick since monday started with fever only vomited once on tuesday but can't get the fever to break and now has a rash?

Better see his MD: Needs to be evaluated, he might need prescription medications/some studies, take care

Pretty typical: This is a story common to many simple viral illnesses. If the kid is playful at times and taking fluids/feeds okay you may be able to give it a day and see if the fever breaks on its own.If the kid is listless, feeding poorly and just wants to sleep, have him seen sooner rather than later.