tikz — How to create rectangle around two nodes which are located diagonally to each other?

Question

I want to have a rectangle that encircles A, AC, and C like the yellow one encircling B, BC, and C (I similarly want one encircling A, AB, and B), but I can't find any way of doing this. When I try like I did for B, BC, and C, the rectangle becomes huge

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes,backgrounds,shapes.misc, positioning,shapes.geometric,arrows,matrix,fit,calc}
    \tikzset{
    buffer/.style={
        isosceles triangle,
        isosceles triangle apex angle=66,
        shape border rotate=90,
        fill=blue!20,
        node distance=5cm,
        rounded corners=60pt,
        opacity=0.6,
        minimum height=6cm
    }
}
\begin{document} 

\begin{tikzpicture}

 \tikzset{venn circle/.style={draw,circle,minimum width=9mm,fill=#1,opacity=0.6}}

\node at (0,0) {A};
\node (ABC) at (0,-2        ) {ABC};
\node (B) at (-2,-3) {B};
\node (AB) at (-1,-1.5) {AB};
\node (AC) at (1,-1.5) {AC};
\node (BC) at (0,-3) {BC};
\node (C) at (2,-3) {C};

\node[buffer]at (0,-1.9){};

\node[draw, thick, rounded corners = 2.5ex, fit=(B) (C),inner sep=3mm, opacity =    0.6, fill = yellow](FIt1) {};

\node[venn circle = green, thick] at (0,0) {A};
\node (ABC) at (0,-2        ) {ABC};
\node[venn circle = green, thick] (B) at (-2,-3) {B};
\node (AB) at (-1,-1.5) {AB};
\node (AC) at (1,-1.5) {AC};
\node (BC) at (0,-3) {BC};
\node[venn circle = green, thick]  (C) at (2,-3) {C};

\end{tikzpicture}
\end{document}

A subquestion I have is whether or not an outline line can be made here to make a black line around the yellow, red, and blue shapes

\begin{tikzpicture}[font=\sffamily\sansmath]

 \tikzset{venn circle/.style={circle,minimum width=9mm,fill=#1,opacity=0.6}}

\node at (0,0) {A};
\node (ABC) at (0,-2        ) {ABC};
\node (B) at (-2,-3) {B};
\node (AB) at (-1,-1.5) {AB};
\node (AC) at (1,-1.5) {AC};
\node (BC) at (0,-3) {BC};
\node (C) at (2,-3) {C};


\node[buffer]at (0,-1.9){};



\draw [line width=35pt,opacity=0.6,blue,line cap=round,rounded corners] (A.center) -- (AC.center) -- (C.center);
\draw [line width=35pt,opacity=0.6,yellow,line cap=round,rounded corners] (C.center) -- (BC.center) -- (B.center);
\draw [line width=35pt,opacity=0.6,red,line cap=round,rounded corners] (A.center) -- (AB.center) -- (B.center);



\node[venn circle = green, thick] at (0,0) {A};
\node (ABC) at (0,-2        ) {ABC};
\node[venn circle = green, thick] (B) at (-2,-3) {B};
\node (AB) at (-1,-1.5) {AB};
\node (AC) at (1,-1.5) {AC};
\node (BC) at (0,-3) {BC};
\node[venn circle = green, thick]  (C) at (2,-3) {C};


\end{tikzpicture}

Answer 1

I was able to obtain the following picture:

For it I reduce your code significantly, see if this is useful for you.

\documentclass{article}
\usepackage{tikz}

    \usetikzlibrary{arrows,backgrounds,calc,fit,matrix,positioning,
                    shapes,shapes.geometric,shapes.misc}
\pgfdeclarelayer{foreground} 
\pgfdeclarelayer{background}
   \pgfsetlayers{background,%
                 main,%
                 foreground%
                 }

\begin{document}
    \begin{tikzpicture}[
venn circle/.style = {circle, draw, thick, fill=#1,
                      minimum width=9mm, opacity=0.6},
     buffer/.style = {
        isosceles triangle,
        isosceles triangle apex angle=66,
        shape border rotate=90,
        fill=blue!20, opacity=0.6,
        node distance=5cm,
        rounded corners=60pt,
        minimum height=6cm},
frames/.style args = {#1/#2}{minimum height=#1,
               minimum width=#2+\pgfkeysvalueof{/pgf/minimum height},
               draw, rounded corners=5mm, fill=yellow, opacity=0.6,
               sloped}, 
                        ]
    \begin{pgfonlayer}{foreground}
\node (A) [venn circle = green]  at (0,0)    {A};
\node (B) [venn circle = green]  at (-2,-3)  {B};
\node (C) [venn circle = green]  at (2,-3)   {C};
\node (ABC) at (0,-2) {ABC};
\node (AB) at (-1,-1.5) {AB};
\node (AC) at (1,-1.5) {AC};
\node (BC) at (0,-3) {BC};
    \end{pgfonlayer}
    \begin{pgfonlayer}{main}
\path   let \p1 = ($(C.center)-(A.center)$),
            \n1 = {veclen(\y1,\x1)} in
            (A) -- 
            node[frames=11mm/\n1] {} (C);
\path   let \p1 = ($(B.center)-(C.center)$),
            \n1 = {veclen(\y1,\x1)} in
            (B) --
            node[frames=11mm/\n1] {} (C);
    \end{pgfonlayer}
    \begin{pgfonlayer}{background}
\node[buffer]at (0,-1.9){};
    \end{pgfonlayer}
\end{tikzpicture}
\end{document}

Edit: In the first version of my answer I miss the point of question. Thanks to @Alenanno, who pointed me to this, I renew it (regardless, that probably the question is duplicate ... it has serve at least as my exercise :-)