I have a 3 month long term break coming up (studying accounting in university). Should I use the time to go for new courses (thinking investment/coding) or use it to get an internship?

Just curious, which year are you in actually? I think as a freshman internships may not offer as much because of the limited knowledge you have in accounting, assuming you are looking at internships in an accounting firm. Hence it might be more beneficial to acquire new skills that can go a long way in helping you in both your university courses and future internships. As a senior, having an internship to put what you've learnt to practice in a practical environment would definitely be helpful and also help you remain competitive amongst your peers in my opinion.

Or of course as other commenters have mentioned, why not do both! the internship experience is definitely valuable since work and school have undoubtedly different environments and 3 months is a fairly long period of time that can surely be dedicated to more than 1 thing:)

Why not do both? Investment courses usually take place over the weekend for one weekend only, with follow ups taking place via fb/telegram or webinars / physical meet up sessions. At least for the one I go for. So it's very possible to have both in your schedule during your 3 month period

Both.

Internship is priority. But you can spend a couple of hours every week reading up on investment.

I definitely would go for the internship, because:

1. It adds to your real work experience
2. One has to learn on the job for the internship in any case!
3. Apart from knowledge, you also get to meet people in the industry you are interested in.
4. No harm earning money on the side.

Besides, you can use your free time to study, especially for subject matters like investment or coding!