Should I buy a crank handle for the series 3

pretty much agree with everyone else. If you have starting issues with the key, then a cranking handle wont help. All you will be doing is working like a starter motor, only slower and more effort.

Ive got a cranking handle for my ambulance, only use it for setting tappets.

Once recovered a 1952 series 1 with the crank handle on steep slope in pennines.
Tirfor winch just kept pulling land anchor through the soft ground so removed plugs and cranked it in reverse gear to get it moving.

Blackburn...that reminds me of the WW2 film...Ice Cold In Alex...where they wound a truck up and over a sand dune on the starting handle.

Nick.

I love that film, set in early ww2 and they drive past a series one in the car park at the end

Best bit...in the bar at the end with the ice cold glass of larger...dont make em like that anymore.

Finally I can start the landy with one or 2 turn of the key with choke, thank you for the advise as I try to check the carburetor, when I unscrew the two cap on top of both carburetor and can hear some air and liquid movement, and the landy can start with no problem, thank you every one.