When somebody is infected with hiv today, can that person transmit the virus after 3 days to somebody else with oral sex?

HIV is Hard to Get: I am always cautious in this setting in that no, this very scenario would be very unlikely to be one of high risk for HIV infection. But, what if one is not newly infected? Or if there were other behaviors not mentioned in the question. It usually takes 14 to 21 days post exposure for full on viral infection and thus contagiousness.

Depends: If the oral sex includes ejaculation of semen into the oral cavity past the tongue and onto the back of the throat and tonsils, absolutely (assuming the acute increase in viral load associated with early infection has occurred). If this was oral sex on a woman the answer is that transmission by that route is highly improbable. Go and get tested.