Water Coming Up Toilet Overflow

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Included photos of water rising to top of overflow tube. We have 3 toilets (two with the pictured Keeney flapper/overflow tube; 3rd is an older PVC & flapper; all 3 do this), and water comes up the overflow tube. It happens when flush begins & flapper opens/handle goes down. Any thoughts? Blocked plumbing vent? All 3 toilets use same fill valves. Thank you.