Peugeot 207 1.4 2007 69000 miles

Hi All New to the forum

My Peugeot 207 has died on me and the mechanic I've used has no idea why so I'm after some ideas.

So went out in the car came home then went to go out half an hour later and it wouldn't start,next day got to the garage where the guy put it on the machine come back with the faults which he then repaired.He changed the plugs as it was showing it was missing on cylinder 3 and also changed the crankshaft sensor.I pocked the car up and it was running lovely so went home.I went out in the car the next day and it went really rough again and started to feel like it was trying to miss fire again, got it home on the sharpish incase it was to break down again and left it on the drive,got in it the next day and again it won't start.It wants to fire but just won't kick into life it sometimes even starts and revs for about 3 or 4 seconds and then just cuts out. I have had it on the machine again since and it is showing lambda senors but apparently this won't cause it not to start so we a both completely baffled by it,

Any help/ideas greatly appreciated

Thanks