How much will a terrible freshman undergradate year affect academic future? I wasn't able to attend university during my freshman year due to personal reasons, so I ended up only barely passing 1 class the whole year (didn't even show up for the finals for all the other classes). My 3rd semester went a bit better, passed half the classes and also passed half of the remaining from my 1st semester. 4th semester went pretty good as I passed all classes with As and during the summer I also passed all the remaining ones from the previous 3 semesters. Right now I'm in my final year and have a fairly good GPA but I'm afraid that my terrible performance during my 3 semesters are going to tank my chances of getting into a MSc and later a PhD program. Is it at all reasonable for me to assume that the admissions boards can look past that since I have clearly shown improvement and have a good GPA?
I'm currently studying in Greece but plan to move abroad (somewhere in the EU) for MSc and PhD.
Note: I was enrolled in all of the core courses during my first 3 semesters I just did not attend class and did not take any exams (no show).
Your older work is far less important than what you have done since. If you have turned it around and people are willing to attest to that in letters of recommendation the first year will likely hurt you very little. I can't say not at all, but not as much as you fear.
For future stuff, focus on the future and on what you have learned enables success there. You are far from alone in this and many students have a slow start. It is good actually, that you found a way to do better. People may well recognize that.
You may have to answer questions in an interview about your first year, so expect that.

Note that the past is past and you can't change it. What you can do now is work so that the past is irrelevant to the future. Make sure you haven't missed any essential knowledge from the early years. Work closely enough with current faculty (projects, research, ...) that they will enthusiastically support your next steps.