Language of all strings that has exactly 1 triple b
It seems you are almost there.  You just need to care substrings with $abb$. One possible way is 
$R.E = (a^{*} (ba)^*(bba)^*)^*bbb(a^*(ab)^*(abb)^*)^*$

Note that I came upon this regular expression from the $FM$ of language you described. So, another way to find $R.E$ is from $FM$ directly.