There seems to still be a fair amount of debate on just how severe the global effects of a super volcano eruption were historically or would be now. The greatest devastation is always localized, so the locations of your three choices matter quite a bit.
A Yellowstone eruption, from the perspective of the Europeans, would most likely take the form of a few years of colder weather and low-yield harvests. This would certainly have an impact in the short term with some starvation and a greater tendency toward uprising, barbarian movement, or chaos, but the long term effects wouldn’t be too serious. North America and its inhabitants would experience some devastation, possibly leading to forced migrations southward. Thus the biggest difference would come during the age of exploration, when Europeans might find far fewer Native Americans occupying North America.
Toba’s distance from Europe is also substantial enough to lessen the direct impact of the explosion, but its proximity to Asia might have interesting effects. Similarly to Yellowstone, Europeans would probably experience several years — maybe even a decade or two — of much colder weather, but it’s unclear how severe it would be. Starvation and increased disorder would happen, but it’s hard to know how much it would affect Europe long term. What might happen, however, is a much earlier migration from Asia into Europe. If Toba’s local effects are devastating enough, you might see eastern invasions happening much sooner, possibly with great long-term consequences.
Campi Flagrei seems to have less historically documented destructive power than the other two, but it’s definitely a more local threat. If it were to experience an eruption on the level of Toba or Yellowstone, you could feasibly see devastation on the continent. With severe ash fall throughout Europe and severe damage in Italy, there might not be a Rome left to sack. Now you might see migrations out of Europe, perhaps east into Asia and the Middle East or south into Africa. This would fundamentally reshape Europe for the rest of history, but how the changes look is really up to you.