Wow...where to start?!?
First, if you read the gage face, it says times 100, so "10" is actually 1000rpm, or revolutions per minute. 20 is 2000, 30 is 3000...you get the gist. 1000 is higher than it should idle, which can explain the run-on you are experiencing after shut down.
Do you have a service manual yet? If not, I would start by getting a copy of that, along with a smaller manual titled, "Practical hints for the maintenance of the Triumph TR3". I know...longer title, but much shorter book! The second is the best if you are just starting to maintain your Triumph with no prior mechanical experience.