Let's start with the easiest one, \mathsf, because that one is actually already there, but the name is \mathss, just like the text switch \ss. If you prefer \mathsf, you can just assign a different name using \let.
For the RSFS and EuScript fonts it is a bit more tricky, because those are legacy TFM fonts, so one has to screw with the low-level font setup to get those to work. For RSFS there is also a Wiki page in the Garden [1] from which you can straighforwardly copy-paste the setups. The EuScript setup works in the same spirit but the TFM names and the skewchar are different.
Keep in mind that the math script alphabet of Latin Modern Math in OpenType is simply the EuScript alphabet but slanted, so there is a high change of confusion! You could either use a different math font or use a fallback to a different font for script. Of course you can just load yet another legacy font for \mathcal as well. In the MWE below I load the Computer Modern script font.
CAVEAT: All of this completely falls apart if you need bold or lowercase.
[1] https://wiki.contextgarden.net/Rsfs Only the section “Improper ways to use the font” applies here. The first section only works in MKII.
% Sans-serif math is there, it just has a different macro name \let\mathsf=\mathss % Load legacy fonts \font\tensy =cmsy10 at \mathstyleface\textstyle \font\sevensy=cmsy7 at \mathstyleface\scriptstyle \font\fivesy =cmsy5 at \mathstyleface\scriptscriptstyle \font\tenscr =rsfs10 at \mathstyleface\textstyle \font\sevenscr=rsfs7 at \mathstyleface\scriptstyle \font\fivescr =rsfs5 at \mathstyleface\scriptscriptstyle \font\teneuscr =eusm10 at \mathstyleface\textstyle \font\seveneuscr=eusm7 at \mathstyleface\scriptstyle \font\fiveeuscr =eusm5 at \mathstyleface\scriptscriptstyle % \skewchar \skewchar\tensy ='60 \skewchar\sevensy='60 \skewchar\fivesy ='60 \skewchar\tenscr ='177 \skewchar\sevenscr='177 \skewchar\fivescr ='177 \skewchar\teneuscr ='60 \skewchar\seveneuscr='60 \skewchar\fiveeuscr ='60 % Allocate family \newfam\calfam \newfam\scrfam \newfam\euscrfam % Assign font families \textfont \calfam=\tensy \scriptfont \calfam=\sevensy \scriptscriptfont\calfam=\fivesy \textfont \scrfam=\tenscr \scriptfont \scrfam=\sevenscr \scriptscriptfont\scrfam=\fivescr \textfont \euscrfam=\teneuscr \scriptfont \euscrfam=\seveneuscr \scriptscriptfont\euscrfam=\fiveeuscr % Make a macro wrapper % Backup original definition of \cal and \mathcal \let\normalcal\cal \let\normalmathcal\mathcal \def\cal{\fam\calfam} \unexpanded\def\mathcal#1{{\cal#1}} \def\scr{\fam\scrfam} \unexpanded\def\mathscr#1{{\scr#1}} \def\euscr{\fam\euscrfam} \unexpanded\def\EuScr#1{{\euscr#1}} \starttext \startformula \mathrm{A} % for Roman font \mathcal{B} % for default calligraphic font \mathscr{C} % for RSFS calligraphic font \mathfrak{D} % for default fraktur / gothic font \mathsf{E} % for default sans-serif font \EuScr{F} % for Euler script calligraphic font \stopformula \stoptext
