All three of these chemicals act as anesthetics when their vapor is inhaled. That means they make a person dizzy, lightheaded, judgment impaired and uncoordinated (intoxication). Inhaling sufficient vapor in a closed space can (and has) resulted in death from coma and inhibited respiratory drive. An additional toxicity of methylene chloride is the fact that one of its metabolites is carbon monoxide (yes, that carbon monoxide). People with heart disease have experienced chest pain while using it (not to mention sudden cardiac death). The EPA has banned methylene chloride for use in consumer paint stripping products. (See:
https://www.epa.gov/newsreleases/ep...ethylene-chloride-consumers-paint-and-coating)
Acetone and MEK are highly flammable and will result in an explosion with the right vapor concentration (and an ignition source). Methylene chloride is less flammable but under certain circumstances will support combustion.
Regular, daily use for several years can cause permanent neurological (brain) injury. But, for most hobbyists, the bigger hazards are the short term intoxication and the fire/explosion hazard.
So, when the label says use in a well-ventilated space, they mean it. Since these substances are absorbed into the body both by inhaling the vapor in air and by direct penetration through the skin, best to use appropriate protection from inhalation (an organic vapor cartridge equipped and properly fitted respirator and non-permeable gloves) or, better yet, if possible, use outdoors.
The hazard is dose dependent so exposure to a small amount of these products or a low concentration in inhaled air is not as hazardous as exposure to large amounts of these products or a high concentration in inhaled air.