Wie sagt man das auf Englisch (UK)?
I studied at a stretch for 3 hrs
would you correct my mistakes?

Sehen Sie eine Übersetzung

Yes, I would correct your mistakes. This sentence does not sound natural. Generally, the phrase that indicates a particular period of time is placed before the phrase "at a stretch".
For example, one could say, "I have fasted for seven days at a stretch". In this example, the time period (seven days) is placed before the phrase "at a stretch".
In your case, you could say "I studied for 3 hours at a stretch".
You could also say "I studied for a 3 hour stretch". I would prefer to use this sentence as I feel that it flows better, but both sentences make sense grammatically.
Both phrases simply mean that you studied for a 3 hour period without stopping.
Hope this helps :)