When was Ezekiel 9:3-7 fulfilled
10/11 years before the destruction of Jerusalem and the 1st temple, king Jehoiachin, king of Judah, surrendered Jerusalem to king Nebuchadnezzar of Babylon. Among those, now exiled Israelites, was Ezekiel, who early in the fifth year (613 BC), of the Jewish king's exile, found himself commissioned by God - Ez, 1:1-3, in no uncertain terms, to act as God's spokesman to the rebellious sons of Israel, to warn them as to the consequences of their rebellious and idolatrous ways.
612 BC - 607 BC
After Ezekiel was told by God, to enact the coming siege of Jerusalem, which was to foresee death by famine, pestilence and the sword, with the remaining inhabitants scattered among the nations, we are told of the appearance (most probably angelic) prior to the actual events, of six men with smashing weapons and of a seventh person, clothed with linen, with a secretary's inkhorn - Ez, 9:2. This seventh man, is told to mark on the forehead, whether symbolically or no, all those detesting the abominations that had come about in their midst, with a view to sparing those so marked, come the death and destruction. The other six men were to slay (also by divine commission, as in the case of Ezekiel), without any pity, all those not so marked, that is to say, man, woman, child, of every ilk within the city and even defile the temple and courts with their bodies - Ez, 9:3-7.
Consequently, to answer the OP's follow up query: It was in 607 BC when this prophecy, against Israel, was fulfilled. It could even be said that this was to foreshadow the 70 AD destruction of Jerusalem and the second temple. It was to be a "type" in other words.
** We know from the historical record that king Jehoiachin was in exile for just shy of 11 years before Jerusalem's destruction. Although, most all historical records place the destruction of Jerusalem and it's first temple, by Nebuchadnezzar, as being in 587/6 BC, this does not jive with the ABSOLUTE date of 539 BC, for the subsequent overthrow of Babylon by the Medes and Persians, and the 70 year captivity of Daniel and his fellow Israelites, which we know ended in 538 BC, which although adding up to 69 literal years, was actually, seemingly to all intents and purposes (the time period being part of further prophecy) 70 x 360 day ...prophetic... years - Dan, 9:1,2. I feel therefore that fulfilled prophecy should trump the apparent inaccuracy of the historical record.
NB - One can be forgiven here when trying to equate a third possible fulfilment of prophecy, to involve Israel, with the coming tribulation as born out in the 24th chapter of Matthew, which incidentally is not only referencing the future tribulation but also the 70 AD destruction of Jerusalem and the second temple, but UNLIKE 607 BC and 70 AD, that will not just involve Israel but will involve the whole world. YES this will involve the marking of foreheads (and even hands) here too, but these will be marks for destruction NOT saving...Rev, 13:15-18. Not withstanding Rev, 9:4, which does indeed involve saving, but there again as in Ez, 9:4, we are talking about the seal of God on the forehead, NOT the mark of the beast.....