All the other sentences were fine.
live in London for 3 years. Personal pronoun and past tense for the verb.
I’m currently work in restaurant, but I hope I change my job as soon as is possible. working in a restaurant... I hope to change...
Both are younger than I. younger than I am//younger than me.
I studied economy. Economics rather than economy
I left in my country a lot of friends. I left a lot of friends in my country.
I studied English in Poland for three years, but I wasn’t a good student.
To be honest I know just a few English words, so when I arrived to London I was shy about my English. past tense; you arrive in a place
After this school I had for one year private teacher. had a private teacher for one year
This teacher was really well, I developed my English so much. good
In last week I’ve been in Poland just for 3 day, and I was running in marathon. Last week I was in Poland for three days where I was running in a marathon.
I was on 305 position in adults and 51 in 18 between 28 years old. I finished 305th in the adults, and 51st in the 18 to 28 year olds.
Now put the sentences into paragraphs.