Division 3 should be easier. If you still cannot solve any problem, then solve easiest problems here. If you still cannot solve most of them, then you are probably missing one or both of the 2 things:

1)Implementation

2)Logic thinking and maths

For the first one, you can become better in it by learning the syntax of the language you are using. For the second one, I guess solving easy maths problems might work. But I don't think you are missing the second one assuming you are in high school or higher.

Atcoder beginners contests .

Try this course. Participate in more Div.2, Div.3 and Educational contests. Visual participation also helps

Also, it is impossible to know how easy/hard a contest will be beforehand since problems are only viewable after the contest starts.

I started competitive on cf 6-7 months ago.Initially i was unable to solve even 1st question of div3. I practiced A and B level questions from junior training sheet and a2oj.com. It helped me lot.You should also try this it might be helpful for you also.