\setuptolerance[verytolerant, stretch]
\setupalign[hz, hanging]
\definefloat[subfigure][subfigures]

\setupfloat[figure][]
\setupfloat[tableleft][]

\setupcaption[figure][width=max, align={hz, hanging}, style=sstf]

\setupcaption[subfigure][way=bypage, location=top, width=max, align={hz, hanging}, inbetween={\blank[1*small]}, style={\smallbodyfont\sstf}, numberconversionset=characters, numberstopper={)}, prefixsegments=4:100]





\starttext

\input tufte

\placefigure[left, here, force]{Maincaption}
{\startcombination[2*2]
  {}{}
  {\blackrule[width=5cm,height=10\baselineskip,color=lightgray]}{caption 2}
  {\newbox\MYscratchbox
   \setbox\MYscratchbox\hbox\bgroup
   \blackrule[width=5cm,height=22.24\baselineskip,color=lightgray]\egroup
   \ht\MYscratchbox=10\baselineskip % fake height
   \box\MYscratchbox}{caption 1}
  {\blackrule[width=5cm,height=10\baselineskip,color=lightgray]}{caption 3}
\stopcombination}

\input tufte

\page[yes]

\input tufte
\placefigure[left, here, force]{Maincaption}
{\vbox\bgroup\hsize105mm
   \hbox\bgroup
     \vbox\bgroup\hsize50mm
       \blackrule[height=22.24\baselineskip,width=50mm,color=lightgray]
       \centerline{caption 1}
     \egroup
     \hskip5mm
     \vbox to 23.24\baselineskip\bgroup % +1 for caption
       \hsize50mm
       \vbox\bgroup
         \blackrule[height=10\baselineskip,width=50mm,color=lightgray]
         \centerline{caption 2}
       \egroup
       \vss
       \vbox\bgroup
         \blackrule[height=10\baselineskip,width=50mm,color=lightgray]
         \centerline{caption 3}
       \egroup
     \egroup
   \egroup % hbox
 \egroup % outer vbox
}
\input tufte

\stoptext