The first equation ($3x + 3 = p$) is correct, but you lost your focus on the second equation. So now Peter's age is $p$ and his sons age is $x$. After three years, Peter's age will be $p+3$ and his son's $x+3$. So "After three years, Peter's age ($p+3$) will be ten years more than twice his son's age" becomes $$ (p+3) = 10 + 2(x+3) $$ When reading this kind of word problems, you just need to keep your head cool, advance slowly and write down exactly what you know.