Space Quest Collection (episodes 1 to 6) is available at Steam today at 2,49 euros (75 percent discount). On mac using Crossover may require to launch the setup application first (see here) to switch the video mode before launching the game. We can also copy files from Crossover to Dosbox or ScummVM.
UPDATE : Space Quest 3 is very slow (music and display) from Steam (uses Dosbox 0.61). By switching the default surface display mode (dosbox.conf file in /Users/<username>/Library/Application Support/CrossOver Games/Bottles/Steam/drive_c/Program Files/Steam/SteamApps/common/Space Quest Collection/dosbox-0.63) to opengl (ok after using Return key) or ddraw display is faster.
However performances are far more greater using boxer, also based on Dosbox (copy sq3 folder from SteamApps/common/Space Quest Collection to the Boxer window). It can also emulate MT32 for music and sounds (requires installing the MT32 rom files in Boxer audio preferences and replacing SNDBLAST.DRV by MT32.DRV in game’s RESOURCE.CFG file).