Petra Ecclestone -- Million Dollar Shopping Spree for 3 Cars!

Petra Ecclestone just dropped a million bucks on 3 cars parked in the driveway of her massive L.A. mansion ... which seems odd for someone who's selling the place and moving to England, right?

Petra -- whose dad Bernie is a billionaire Formula One honcho and her hubby James Stunt who is also a billionaire -- is now the proud owner of a $600K Lamborghini Aventador convertible ... a $422K Rolls Royce Phantom ... and a Range Rover Autobiography that goes for $190K.

Celeb consultant Richie Rich hand delivered the wheels from Exotic Euro Cars.

TMZ broke the story ... Petra went to 5 L.A. realtors and told them she wanted to sell the crib for $110 million -- making it the biggest sale in L.A. history -- because she was going back to the UK.

But we're told she's now decided the estate is worth $150 million, so until she gets it she's staying put.