first off, speak to your trainers and let them know you would like to fight as lots of people just go to get fit.

Everything is still very new as you've been doing it 3 weeks, just start to make small adjustments to clean up your diet if it isn't already, that might just mean switching to wholemeal bread/pasta/rice rather than white.
Make sure you're getting enough fluids and rest. It'll just be easier and healthier for you in the long run. As you get fitter you'll be able to refine your programme, add in extra training in the gym and away, add in sprint work etc.

Thing is, for now just enjoy the experience of your knew sport and seeing how your body develops. You don't need to rush and risk doing too much & get injured.

Watch fights on tv, watch em live. Start to look for mistakes guys make & there will be many.
I read in a good boxing book - there's always a reason you got hit - that might be you dropped your hands, telegraphed a shot & you got caught etc

Best of luck and welcome aboard.