Regarding the fraction of satisfiable 3-CNF formulas in $n$ variables, it is widely believed that there is a phase transition that occurs depending on how many clauses there are compared to the number of variables. To be precise, it is conjectured (but not yet proven) that if tharethere are more than $\alpha n$ clauses, then the formula is almost surely unsatisfiable, and if there are less than $\alpha n$ clauses then the formula is almost surely satisfiable (where $\alpha$ is around 4.2667). This phase transition has been established for $k$-SAT when $k$ is large. See Proof of the satisfiability conjecture for large $k$ by Ding, Sly, and Sun.