Which Turing machine problems are Decidable?
You can simulate a given Turing machine to a given number of steps. This allows you to decide Problem 1 and Problem 3, by running $M_N$ on the empty type for $N$ steps (or fewer, if it halts earlier).

Regarding Problem 2, assuming that $N$ is in the language if


$M_N$ halts on the empty tape, and
$M_N$ doesn't halt within the first $N$ steps,


then you can use this language to solve the halting problem (using the same effective enumeration). Given $N$, in order to determine whether $M_N$ halts on the empty tape:


Run $M_N$ for $N$ steps. If it halts, answer Yes.
Otherwise, use Problem 2 to determine whether $M_N$ halts on the empty tape, but not within the first $N$ steps.


Hence Problem 2 is undecidable.