documentclass[margin=5pt, tikz]{standalone}
usepackage{tikz}
usetikzlibrary{calc}
usetikzlibrary{angles, quotes, babel}
usepackage{amsmath, amssymb}
begin{document}

begin{tikzpicture}[x=1cm, y=1cm, z={(4mm,3mm)}, scale=1.5, font=small]
pgfmathsetmacroa{2}
pgfmathsetmacrob{2.5}
pgfmathsetmacroc{3}

pgfmathsetmacro{r}{0.5*sqrt(a^2+b^2+c^2)} %

pgfmathsetmacro{AC}{sqrt(a^2+b^2)} %
pgfmathsetmacro{PA}{0.5*AC} %

pgfmathsetmacro{h}{sqrt(r^2-PA^2)} %

% Pyramid
draw (0,0,0) coordinate[label=below:$A$] (A)
--(a,0,0) node[midway, below]{$a$} coordinate[label=below:$B$] (B)
--(a,0,b) node[midway,right=2mm]{$b$} coordinate[label=$C$] (C);

draw[densely dashed] (C)--(0,0,b) coordinate[label=left:$D$] (D)--(A);
draw (A)--(0,c,0) node[midway, left]{$c$} coordinate[label=$S$] (S);

 % Diagonals
draw (A)--(C);
 draw ($(A)!0.5!(C)$) coordinate coordinate[label=below:$P$]  (P) -- ($(S)!0.5!(C)$) node[midway, left]{$h$} coordinate[label=$M$] (M);
draw (D)--(B);

 % Point M
draw (S)--(M) node[midway, above]{$r$} -- (C) node[midway, above]{$r$};

draw[densely dashed] (D) -- (M);
foreach P in {A,B,C} draw (M) -- (P);

node[anchor=north west, align=left, yshift=-7mm] at (A){$begin{array}{l}
a = a ,~  b = b,~  c = c \
|AC| =  sqrt{a^2+b^2} = AC \
|PA| = 0.5 |AC| = PA \
h = sqrt{r^2- |PA|^2} = h  \
r = 0.5 sqrt{a^2+b^2+c^2} = r
end{array}$
};

shade[ball color = gray!40, opacity = 0.4] (M) circle[radius=5mm] node{?};
end{tikzpicture}

end{document}