Read this

As we all know div3s are very hard, you have to know a lot of complicated things to start giving contest. Complicated concepts like arrays loops conditions should be in your bag xD.

well jokes apart, just give contest there is nothing to loose for you, you will know what div3 is like after giving contest.

There's no need to wait specifically for div 3 contests. Yes, the problems are somewhat easier, but div3 contests are also infested by an abnormally large number of smurfs (blue+ guys registering for the contest with their second newly created grey accounts). The "trusted participants" filtering at least removes some of them from the scoreboard, but rating update calculation still takes smurfs into account and they eat your rating. If you are concerned about your rating, then div 3 contests are not necessarily a good choice. You won't lose much by just participating in div 2 contests and you will get up to speed much faster.

As for the topics that may show up in div 2 / div 3 codeforces contests, check this blog: https://mirror.codeforces.com/blog/entry/80049 (hint: codeforces problems A-D are mostly adhoc and it's very uncommon to see any advanced data structures or algorithms in them).

And if you want to have fun solving easy problems during a real contest, then AtCoder ABC contests and CodeChef starter contests are a good alternative. Ironically, these platforms will allow you to get familiar with more advanced algorithms in an easy way (the problems are not designed to be very difficult puzzles).

nothing

You can give a contest and know it for yourself. Usually, the first 3 problems in div 3 don't require any prerequisites (don't take my word for it since it's Codeforces). Next 1 — 2 problems may require idea of greedy, binary search, basic Number theory or basic graph algos like dfs or bfs. (It's still majorly observation reliant)