The trio also took pole last year in a race won by Germany's Nico Hulkenberg, New Zealander Earl Bamber and Britain's Nick Tandy.
Torrential rain in Thursday night's third and final session meant Jani's time stood, with the number one Porsche driven by endurance world champions Mark Webber of Australia, Germany's Timo Bernhard and New Zealander Brendon Hartley completing the front row.
The final session was red-flagged for 57 minutes due to the conditions.
Porsche and main rivals Audi have each entered only two cars instead of three in the main LMP1 category for cost-cutting reasons in the wake of parent company Volkswagen's diesel emissions scandal.
Hulkenberg is in any case otherwise engaged racing for his Force India Formula One team in the European Grand Prix in Azerbaijan, a race that clashes with Le Mans due to the expansion of that series' calendar to a record 21 races.
Bamber and Tandy are driving Porsche 911s in the GTE category.
Toyota took the second row of the grid with their number six and five TS050 Hybrid cars.
Once-dominant Audi start fifth with Germany's Andre Lotterer, France's Benoit Treluyer and Switzerland's Marcel Fassler in the number seven entry.
Ford, back at Le Mans 50 years on from their 1-2-3 finish in 1966, led the GTE Pro category with Germany's Dirk Mueller in the Chip Ganassi racing car.
The 84th edition of the race starts on Saturday at the circuit in western France with Hollywood actor Brad Pitt the honorary starter.
Porsche, who returned to Le Mans with a works team in 2014 for the first time in 16 years, are the most successful manufacturer with 17 titles.
(Reporting by Alan Baldwin in Baku, editing by Nick Mulvenney)
