About to graduate and looking for full-time IT job; how to mention my internships experience on my resume?

To which skill level should i apply?

First decide what role and position you want (full stack, home automation, or something else you really like).

Then, I would suggest you focus on Junior positions, as you are just finishing your degree and it's more likely/adequate to land such role given your experience.

You could go for Senior roles, but surely it will be harder to land an offer as a Senior right out of University.

Should i put 3 years experience in my resume?

That would depend on the role you seek to apply to.

If it's full stack, and you have, say 2 years on it, specify it that way. If it's home automation, and you have 1 year experience on it, that is what you should include on your resume.

If it's something different, that you don't have experience on, then it would be unprofessional to mention you have experience on the matter.

Whatever you decide, focus on the experience that relates most to the job. You can also include other relevant or interesting experience you have (as it would be your first "official" job with a degree), but it would be best to highlight those that relate more strongly to the role you seek.