The language of set of all strings over {a,b}* containing at least 3 a's is regular.
RE is (a+b)* a (a+b)* a (a+b)* a (a+b)*.
If a language can be simulated by a FA, then it can be definitely simulated by a PDA.
CFG for the language would be:
S $\rightarrow$ AaAaAaA
A $\rightarrow$ aA|bA|$\varepsilon$