Start msys from the start menu. Then try:
pacman -S 3ds-dev
I get the following log:
$ pacman -S 3ds-dev
:: There are 13 members in group 3ds-dev:
:: Repository dkp-libs
1) 3ds-examples 2) 3ds-pkg-config 3) citro2d 4) citro3d
5) devkitarm-crtls 6) libctru
:: Repository dkp-windows
7) 3dslink 8) 3dstools 9) devkitARM 10) devkitARM-gdb 11) general-tools
12) picasso 13) tex3ds
Enter a selection (default=all):
resolving dependencies...
looking for conflicting packages...
Packages (15) devkitarm-rules-1.2.0-2 tool-dlls-1.0-1 3ds-examples-20200716-2
3ds-pkg-config-0.28-5 3dslink-0.5.2-2 3dstools-1.1.5-1
citro2d-1.4.0-1 citro3d-1.6.1-1 devkitARM-r55-1
devkitARM-gdb-9.2-4 devkitarm-crtls-1.1.0-1
general-tools-1.2.0-17 libctru-2.0.0-2 picasso-2.7.0-1
tex3ds-2.0.1-2
Total Download Size: 53.83 MiB
Total Installed Size: 256.03 MiB
:: Proceed with installation? [Y/n]
:: Retrieving packages...
error: failed retrieving file '3ds-examples-20200716-2-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file '3ds-pkg-config-0.28-5-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'devkitarm-rules-1.2.0-2-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'devkitarm-crtls-1.1.0-1-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'libctru-2.0.0-2-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'citro3d-1.6.1-1-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'citro2d-1.4.0-1-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'devkitARM-r55-1-x86_64.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'tool-dlls-1.0-1-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file '3dslink-0.5.2-2-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file '3dstools-1.1.5-1-x86_64.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'devkitARM-gdb-9.2-4-x86_64.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'general-tools-1.2.0-17-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'picasso-2.7.0-1-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed retrieving file 'tex3ds-2.0.1-2-any.pkg.tar.xz' from downloads.devkitpro.org : Could not resolve host: downloads.devkitpro.org
warning: failed to retrieve some files
error: failed to commit transaction (invalid url for server)
Errors occurred, no packages were upgraded.