If you're willing to customize your installation, you can do so much better. If you make it through this process, then you'll have a good working understanding of DOSBox and be able to optimize performance in other games.
For MM4-5, run the install (double-click dosbox.exe, type 'mount c: ".."', hit enter, type 'c:', enter, type 'install', enter). Select Roland Sound Canvas for music and Sound Blaster 16 for audio. Set up VirtualMidiSynth with a good soundfont or two (you can find soundfonts made from ROM dumps of the Sound Canvas, or use an entirely different soundfont--just use what sounds good to you) and specify it as your MIDI device. This will give you the best possible music. If you experience a crash when saying goodbye to the barkeep at the Vertigo tavern, reconfigure the Sound Blaster in both the install program and your dosbox configuration to use IRQ 5 instead of the default 7.
Disable scaling in your GPU options, or set it to "centered". If you have an Nvidia card, don't tell it to defer to your display. If you have a 1920x1080 or larger display, you need a higher scaling multiplier than the included DOSBox build offers. Download an SVN build like the Daum's Cafe build which includes Normal4x in order to get a sharp fullscreen image which nearly fills the entire screen vertically. Normal4x simply quadruples the pixels on each axis--no blurry mess.
Here's the only downside to having perfect audio and graphics: without the stock DOSBox build, the CD version won't run. Use the non-CD version. You lose out on most of the voice-acting, but I think most people would consider the trade-off worth it. There's no other downside.
This advice--reconfiguring sound using the game's install (not GOG's configurator) to make use of VirtualMidiSynth (or MUNT), and using an enhanced SVN build of DOSBox for better image scaling support--applies to most DOS titles on GOG. GOG's stock configurations are simply abominable.
I can't remember the sound config options offered by MM3, but your aim should be the same--to get DOSBox to look to VirtualMidiSynth. The video configuration will be identical.
I also recommend my autohotkey script which allows one-key strafing for all three games. You can find it
here.