A process of copolymerizing propylene, ethylene and at least one alpha-olefin having from 4 to 10 carbon atoms in the presence of an olefin polymerization catalyst in three cascaded polymerization stages comprising the steps of: (i) copolymerizing propylene, ethylene and the alpha-olefin having from 4 to 10 carbon atoms in a first polymerization stage by introducing a first monomer mixture consisting of propylene, ethylene and at least one alpha-olefin having 4 to 10 carbon atoms, and said olefin polymerization catalyst into the first polymerization stage under conditions to polymerize propylene, ethylene and said at least one alpha-olefin on said olefin polymerization catalyst to produce a mixture comprising particles of a first semicrystalline copolymer of propylene and a first fluid reaction mixture; (ii) withdrawing a first stream of the mixture comprising said particles of the first copolymer and said first fluid reaction mixture from the first polymerization stage and introducing it into a subsequent second polymerization stage; (iii) introducing one first additional monomer mixture consisting of propylene, ethylene and optionally at least one alpha-olefin having 4 to 10 carbon atoms into the second polymerization stage to copolymerize propylene, ethylene and the alpha-olefin to produce a mixture comprising particles of a first copolymer mixture of said first semicrystalline copolymer and a second semicrystalline copolymer and a second fluid reaction mixture; (iv) withdrawing a second stream of the mixture of said particles of the first copolymer mixture and said second fluid reaction mixture from the second polymerization stage and introducing it into a third polymerization stage; (v) introducing one second additional monomer mixture consisting of propylene and optionally ethylene into the third polymerization stage conducted in gas phase in a fluidized bed to polymerize the monomers in said second additional monomer mixture optionally with residual alpha-olefin remaining in said second stream, in the third polymerization stage to produce particles of a second copolymer mixture of said first copolymer mixture and a third semicrystalline polymer of propylene where the particles of the second copolymer mixture are suspended in a third fluid reaction mixture in gas phase; (vi) withdrawing a third stream comprising the second copolymer mixture and the third fluid reaction mixture from the third polymerization stage. (i) copolymerizing propylene, ethylene and the alpha-olefin having from 4 to 10 carbon atoms in a first polymerization stage by introducing a first monomer mixture consisting of propylene, ethylene and at least one alpha-olefin having 4 to 10 carbon atoms, and said olefin polymerization catalyst into the first polymerization stage under conditions to polymerize propylene, ethylene and said at least one alpha-olefin on said olefin polymerization catalyst to produce a mixture comprising particles of a first semicrystalline copolymer of propylene and a first fluid reaction mixture; (ii) withdrawing a first stream of the mixture comprising said particles of the first copolymer and said first fluid reaction mixture from the first polymerization stage and introducing it into a subsequent second polymerization stage; (iii) introducing one first additional monomer mixture consisting of propylene, ethylene and optionally at least one alpha-olefin having 4 to 10 carbon atoms into the second polymerization stage to copolymerize propylene, ethylene and the alpha-olefin to produce a mixture comprising particles of a first copolymer mixture of said first semicrystalline copolymer and a second semicrystalline copolymer and a second fluid reaction mixture; (iv) withdrawing a second stream of the mixture of said particles of the first copolymer mixture and said second fluid reaction mixture from the second polymerization stage and introducing it into a third polymerization stage; (v) introducing one second additional monomer mixture consisting of propylene and optionally ethylene into the third polymerization stage conducted in gas phase in a fluidized bed to polymerize the monomers in said second additional monomer mixture optionally with residual alpha-olefin remaining in said second stream, in the third polymerization stage to produce particles of a second copolymer mixture of said first copolymer mixture and a third semicrystalline polymer of propylene where the particles of the second copolymer mixture are suspended in a third fluid reaction mixture in gas phase; (vi) withdrawing a third stream comprising the second copolymer mixture and the third fluid reaction mixture from the third polymerization stage.