Piastri moved into the path of Leclerc whilst trying to avoid Ricciardo, which sent the Ferrari driver onto the grass in evasive action.
"I nearly shunted with Oscar," the Monegasque reported over team radio in the aftermath.
What happened between the drivers in FP3?
Ricciardo appeared to be caught in the crossfire between Piastri and Leclerc, whose run in the on the exit of the second Lesmo Curve was not their first of the lap.
Leclerc had inadvertently blocked Piastri at the first corner, forcing the Australian to abort his lap.
A few turns later, the two nearly came together again.
Piastri was looking to avoid the oncoming Ricciardo and aimed to do so by moving towards the right-hand side of the track.
Seemingly unknown to the McLaren driver, Leclerc was hurtling down the straight on that side and had to take evasive action to avoid the 23-year-old.
Both Leclerc and Piastri have been summoned to the stewards for allegedly impeding Ricciardo. The RB driver will also see the stewards to explain his version of events.